For a 600-620ish HP (Flywheel) combo with just under a 600 lift cam and only being spun to 6300, how much if any ET lost would there be with using a 1 3/4" header vs 2"? Car is 3150lbs. TTI headers in a A-Body. TTI recommends their 1 3/4 for 300-475HP, and their 2" for 500-750HP, not sure if that's flywheel or rear wheel.

I'll say 20-25HP less with the small headers on a legit 600+ HP big block.

That would be flywheel power tri mentions.

Go with the largest header you can. (Within reason)

Years ago I dyno tested a pump gas 426 M.W. motor with a set of Hooker A body 1 7/8 pipes with a 3/5 collector, the dyno owner made a set of dyno headers for the Mopar wedge motors so he could test them in the future with out borrowing mine. His where 2 1.4 pipes 25 inches long with a four inch collector, I was sure that it would kill the HP and torque down low under 5000 RPM but that stupid motor gained power through out the RPM range (we tested it from 3500 to 6500 RPM)
On all A body with BB that I have raced they all have pick up ET and MPH with bigger pipes and open exhaust.
Hedman makes a nice set of 2.0 fender well headers for A body with BB that fit better than the older Hookers do
Make the rest of exhaust system as big as possible also, 3.0 is way better than 2.5, way better
If you want a set of TTI chassis headers look at the 2.0 to 2 1/8 step headers, if they offer them, along with a 3.0 exhaust system

Could be zero difference. If it was me I'd go with the 1 3/4 headers just so you can work on the car easier. I've tested different headers on the dyno and a really good dyno header will lose some power if it is too small. But chassis headers are not dyno headers. Look at the TTi headers and you'll see that they have sharp bends right off the ports. So you are already losing power with that first bend. Since even the big headers are going to be hurting the power over a set of dyno headers I'd just go ahead and use the smaller ones that fit better.

If you don't like that argument then think about the FAST guys who are running in the 10's with cast iron manifolds. Surely the 1 3/4 header is better than a cast iron manifold and those guys are making more power and going faster than you are. In fact, I bet you could use a Tri Y header in a BB A body car and still run 10's without any trouble at all.

So many guys dont see the difference of the dyno
header than a non dyno set.. they tend to think
there is no change in power.. there is.. I'm glad
you said that
EDIT
that first turn is a killer when its tight up
against the head