[GRX]

"How accurate do those drill holes need to be?....+/- 1/32" or tighter?....good for a machine shop or this possible with standard 'home shop' tools?"

Hole locations must be very accurate to ensure proper gasket seal. Very possible at home with good drill skills. I have done more on the workbench than a mill.

Best method is driving alignment dowels from the block into the heads to place gasket at proper location. In lieu of that you could use some head bolts with washers.

Lightly tap with hammer using your nice sharp center punch. Once you are confident those are centered with holes in head gasket, hammer harder to give yourself a nice divet to drill on.

As for hole sizes, start with 1/8" on both straight holes (exhaust side) and angled (intake side). You can leave it at that if desired. If confident you can open them up to approximate factory sizes. 1/4" on the straight, and 3/16" on the angled.

Once you experience the 400 you won't want to go back to a 350. The 3.75/4.125 stroke/bore combination can make some decent power. Interesting to note that the Pontiac 400 is virtually the same (3.75/4.120) yet has a very different power curve because of their head and valve design. And unlike the SBC 400, they are internally balanced. But that is getting off topic.