Coming right up.
These are going on a 72' 318 shortblock with summit cam and REVERSED PISTONS.

https://www.summitracing.com/parts/sum-k6901/make/plymouth#overview
218/228 114 .470 (with 1.6 rockers)

I believe the results will be very worthwhile. The mistake people made back in the day was they "Hogged out" everything thinking about those big Hemi HP numbers. Removing the rough as cast finish, blending parting lines and rounding off squared valve guide edges can only help. Are you gonna smooth the combustion chamber surface finish? Some say leave it rough to promote mixing and others say smoothing it takes away carbons ability to cling and build up. I polished my J head combustion chambers and exhaust bowl/ports because I had a cool Dremel and was having fun. Left the intake ports as done with a course cartridge roll. But I still have not installed my heads on anything.

Years ago I saw a pair of cast iron Olds 455 heads originally hand ported by Joe Mondello way way back. The combustion chambers were so polished and smooth it was amazing to see the effort that went into them.

I like what you are doing. Don't have to drop 7 grand to build a motor to have fun with if you scrounge the junkyards and start with decent core parts. Are you planning to run headers or manifolds? What is the rest of the planned combo? BTW who did the flow testing on your heads?

In the over 40-years of porting motorcycle 2 & 4 stroke singles and multi cylinder heads (and matching engine cases to 2-stroke cylinders) I always went for a smooth polished finish to maintain flow. Velocity and maintaining it has always given the best results. Hogging out ports and passageways always results in a drop in velocity and that is not what makes power IMO. I have had lots of dyno time and it was easy to see the increase in power with different porting (finish) methods with a single cylinder 2 or 4 stroke and a smooth surface and smooth flow could show an increase of 5% to 10%. Here's one of my 906 heads that will be on my 440 engine............if a rough surface was better than I'd be putting corrugated tubing headers on........just joking

Hers is some Small block J heads I did myself. The intake ports and bowls are not polished but left as is by the cartridge rolls. Combustion chambers, exhaust bowls and exhaust ports are polished. Intake side is gasket matched and I removed a little of the pushrod pinch. I did not gasket match the exhaust side but by removing the cast rough finish it did enlargen the port window slightly. But I've exited the car hobby recently. These are just stored in my house now. I rounded and contoured the valve guide bosses. Steve Duclich did an article on his porting of J heads around 2001. He had IMPRESSIVE flow numbers but he really removed ALOT of the valve guide boss which I did not even begin to do.

Roll up your windows!

A12- That is some nice work. Its also nice to see grassroots low buck projects. It is possible to have a set of used OEM heads that have good guides and seats. In this case a home porting job is a FANTASTIC option. Way way cheaper than even the cheapest aftermarket head and plenty good enough to make moderate power and torque. The culture of the hobby has taken a Sunday morning TV engine build turn where every engine build- even daily driver lower power street engines get aluminum heads, roller rocker arms and forged pistons.

I love this you guys...some cool DIY stuff.

So my first time through on the factory iron took me to where 2boltmain ended. But...heck...can't just stop there, right? LOL

Well, I went a step further and focused on removing that valve guide boss casting leftover which connects the main part of the guide to the roof. It just seemed to be the only major 'obstacle' left in the path of airflow. My main goal here was to create a more seamless roof flow path.

Here is what I ended up with (4 attachments only), but I'll do a follow-up post that shows the flow bench differences as these heads were done by the same guy, on the same bench. So it's not so much the numbers hit that I was after (although there is no denying it that it's good to see impressive values), instead I was focused on the CHANGE between the two sets of numbers.

OK, so here are the flow charts, and the chart LEGEND you see is as follows:

1) AP1 - these iron heads, 1st set of changes
2) AP2 - these iron heads, 2nd (and last) set of changes I described above
3) Ported 596- these are magazine article based, may have been a Dulcich porting articles, it's been a while though so I can't remember
4) Performer RPM - someone's flow numbers for these heads, out of the box
5) Ported W2 - someone's flow numbers for these hads, following porting work

My conclusion was:
1) you can improve on the INTAKE, but with stock heads the EXHAUST will hold you back a bit
2) the numbers themselves appear to be somewhat optimistic (peak flow), but down low these were pretty consistent with what others have seen, take a look at the feedback I got from our Moparts folks on my post HERE

My question, has anyone done a comparison between the J head and Magnum head ported the same way to see which head is better? Even big intake and small intake valve?

I have not done testing myself, but from what work others have done It seems in an all out race effort the J heads are a little better simply because of the intake runner volume. Factory magnums are limited by the pushrod hump.

For a low bucks daily driver build I would argue the magnum head is better. Look at the exhaust flow numbers in the video OP was able to achieve- over 200 cfm at .500 on the exhaust. That combined with modern closed detonation resistant chamber make them my choice for a street build to have some fun with. More static compression can be tolerated on pump gas with the magnum heads and properly set up quench.

Of course you could get pistons and machine work done on the J's to set them up with quench, but for a lot more money unless you happen to own the machine shop.

Just my two cents. I recognize there is more than one way to skin a cat and more than one way to go about it.