If a block is "hard-filled" to the bottom of the freeze plugs. Do you fill the radiator completely full as you would on a street car?

Seems like my 400 wants to burp and bubble water out until the radiator is half full. TO me this does not make sense, because the cooling system should be sealed..

Unless I'm a dummy and I have a head gasket causing #3 to have problems. (130psi cranking).

But if you look at the exhaust in the video, you don't see water pouring out, smoking, etc.. other than a little alcohol bypassing #3 :)

YouTube- 1968 Camaro Drag Car - Alky SBC Idle/Cam

That's a .060 over stock 400 block with 14:1 compression? Take it apart. It might be junk. Alcohol motor should run extremely cool. If it uses water it might be cracked. Milkshake in the oilpan?

I did my 400 block and never had an issue like that

You have a problem somewhere. The radiator should stay at a constant level, roughly a couple of inches below the cap. It could be a cracked block, head, head gasket, etc.. The filler will not have any affect on the level of the fill, just the volume of the fill.

The hard filled block threw my line of thinking off initially. But once I realized it's still a sealed cooling system, just with smaller overall water volume- I figured you troubleshoot it like any other SBC.

Good news is I think I found the problem. Just did a cranking compression test on all 8 since I took the headers off.

Here is the run down:
1 = 205
3 = 131
5 = 132
7 = 187
2 = 190
4 = 230
6 = 175
8 = 223

Notice #3 and #5. They are both around 132. I started the troubleshooting because my heat gun showed 200 degrees less heat in #3 and initially low cranking PSI. I couldn't test #5 until I had the header off.

Keep in mind this is a FRESH engine. 0 passes, and I have only ran 6 gallons of methanol through it dialing in the fuel pressure and setting valves/timing.

I'm thinking
#1 the builder didn't break the rings in on gas
#2 head gasket issues between #3 and #5.

He checked the block and did all the machine work at a race shop so hopefully it's only a head gasket.

I'll know in a few minutes when I pull the drivers side head off.

Ditto on the block filler, shouldn't have any consequence towards your issue. One thing to keep in mind though is Moroso filler states right on the paperwork that it is NOT compatible with anti-freeze, so it's a race only item. Hard Blok block filler is compatible with anti-freeze, that's what I used to short fill my 400 also.

Bad news.. pulled the head on the 421 today and found the coolant problem. #3 and #5 have about a 2" crack from the top of the deck inline with the coolant jackets.

This explains why I was loosing water and #3 and #5 only built 130 PSI cranking compression. Doesn't sounds like a freshly built 750HP 421 to me. THis is what I was told when I bought it, no passes and had only been broken in.

Hopefully you can get your money back!