When doing a 470 stroker in a 400 block I noticed a 6.70 or 6.535 rod can be used. Is there an advantage to using one over the other?

Both will work fine but the 6.70 will give you a shorter compression height for the pin and a slightly lighter bobweight and slightly higher (less rod angularity/side wall loading at high RPM) rod ratio.

Both 6.535" and 6.70" are common BB Chevy aftermarket lengths, given the 2 I'd personally go 6.70". 6.535" is the most common length for the 4.25" Stroke Low deck since it happens to take the same piston CH as a 451" with a standard 6.76" Mopar rod.

Use the rod that allows you to run the best piston.

Is there anything like a consensus as to "what is the minimum compression height that gives reasonable life expectancy?"
We know that you can make a piston very small by tightening the ring-to-ring vertical distance, moving the entire stack up, reducing pin diameter and boss thickness, but when is this just a ticking bomb?
Do extended tangs help stability with a short rod ratio, or just add weight?

Until we know the piston is safe, the rod length has to be a compromise. Above about 1.25" CD, I'd use the longest rod I could.

I would talk to my preferred piston maker and go from there. I am betting though that a compression height of 1.20 or more, maybe up to 1.32 will be just fine with out much difference in performance.

Goes without saying: a turbo or nitrous piston has to be bigger, top ring farther down, bigger pin.

The longer the rod, within reason, the less side loading into the cylinder wall, especially on stroker motors
My first low deck stroker motor (pump gas)I built back in the early 2000s, was a 400 block with a 4.25 stroke and a set of CAT brand BB Chevy type rod that was 6.800 long center to center on each end. I had a set of Ross dished ppistons made to get 9.25 to 1 compression ratio on a set of iron 906 heads with 84.00 CC chambers, that motor flat rocked with those parts Those pistons with out the wrist pin or rings weighed 456 grams each
Some of the nay sayers on here said it couldn't be done, but I was stupid and didn't know that it couldn't be done so I did it any ways
Go long as possible on the rod length
The longer rods really helps on make the pistons really light weight which helps the acceleration and deacceleration
The only down side I saw was that those pistons would come out of the bottom of the bores which you could see some slight scuffing on the pistons skirts when I took it apart to put in a 4.300 stroke crankshaft later to raise the compression ratio up to 10.29 to 1 after I swapped the heads to a set of aluminum Eddy RPM heads. I couldn't measure any wear on the skirts though so no damage, no foul
I have built two bracket race 400 block strokers since then one with a 4.250 stroke with a 6.700 long H beam steel rod and the other one has a 4.300 stroke crankshaft in it with the same 6.700 rod length.
I have bought some used 400 stroker motor parts that included a 4.250 stroke Winberg crank with a set of Crower billet steel 1000 HP H beam rods that are 6.535 long, which I will use in the next 400 stroker motor I build

When using a longer rod doesn't the Pistons get shorter which requires having to use a ring support rail which causes problems,

I don't know if it true just stories, and what off the shelf piston would work using the longer rods 6.700
Only asking because next build will be 470 or 512

with a conventional BBC .990 pin the need for a support rail for the oil ring (which in itself is no big deal) usually comes into play around 1.32"-1.30" of Compression height when using a conventional set of Top, 2nd and 3 piece oil rings, but this requirement can vary widely simply by the design, width and center to center spacing between the ring packs.

Shorter rod ratios and higher pin placements tend to incurr more piston "rock" at TDC and BDC which puts more emphasis on block rigidity and skirt design to maintain good seal and longevity. On a street motor with moderate peak torque and HP RPM targets it's really not a deal breaker but on higher RPM race motors you can get to a point where you might consider sacrificing stroke to gain RPM depending on the combination. the 470" is a great race engine, it's 10% smaller displacement than a 511 but with the right combo you can trade off the cubes for a 10% higher rev cieling and make the same HP, but on the street with mid 3 gears and a street converter (designed to use and not side step the abundant low-mid range torque) the 511 is a great way to go.

Maybe some of us "over-think" it in practical terms, particularly when you consider bowtie guys routinely put 4.25" cranks with only 6.135" rods in standard 454 chevy blocks with only 9.80" deck blocks, some of them use "long" 6.20-6.3" 'Hybrid' BBC width/SBC rods with 2.100" journals and .927" SBC pins.

The oil ring support rail is not a problem if installed correctly.
The issue I found on my low deck 499 engine is there were a slight burr where the pin hole was machined that kept the oil ring support from sitting flat in the ring land, and made the oil rings tight. De-burr the pin hole in the ring land and everything was fine.

FWIW, I run Chevy Eagle 6.385's in my heap.........

I also have the Eagle 6.385" rod in my Roadrunner with a 469. I think it is the best combo. This car used to have a 451 with a 6.760" rod. I like the 469 a lot more.

These little tightly wound 470's are a beast...........

If you haven't run both rods in both combos...... how can you make an informed comment?

I’m going to tell you this, you won’t find 5hp difference between the two different rod lengths.

Build the most common combo 6.385” rod and you won’t be disappointed.

Here is another combo 6.535” rod, 3.90” stroke, 4.375” bore (400). 1.490” piston ch.

Have you actually checked?..... If there is less than 5hp difference (disregarding wear & reliability issues) How can you promote one over the other?...

Lots and lots of people have run short rod vs. long rod tests over time. The general consensus is that it doesn't matter most of the time. The OP didn't tell us much about the engine he wanted to build but it is doubtful that he is a serious professional since if he was then he wouldn't need to ask that question on a web forum.

About the only place I know of where people worry about rod ratio is some circle track engines. Even there, some drivers like short rod engines to get them out of the corner while other like long rod engines for the straights. So I'm not sure there is even a consensus about which to use.

Personally I use a longer rod whenever I can just because I like to keep the piston away from the crankshaft. Other guys like short rods and they seem to do just fine.

For the average guy the advice I gave earlier in the thread should cover it. That is, just pick the piston you want to use and then buy the rod that connects that piston to the crank. You don't need to put anymore time into it than that.