Hey Kablooie, welcome to the forum! I'm glad to be of some help so soon on here.
400 heads on a 350 block is a great way to increase your midrange and top end power. The swap is a direct bolt - on deal, you just need to be sure to use 400 head gaskets when you do the swap. 400 heads have steamholes drilled into them for heat dispertion, and 350gaskets do not have provisions for these steamholes. 400 heads have 2.02" intake/1.6" exhaust valves on nearly all applicationsversus the 1.94"intake/1.5"exhaust valves found on most 4-bbl 350 heads andI believe 1.78"intake/1.35exhaust valves found on most 2-bbl 350 heads. This in part with generally slightly larger port runner size is what gives you the added midrange and top end. The downfall of course will be a slight decrease in fuel mileage.
Also, another word to the wise on 400 heads....they are prone to cracking in the combustion chamber...generally in the center two cylinders between the valves. If you are uncertain as to if your set are cracked or not, a compitent machine shop can tell you if they are or not. And don't let them tell you that the heads are junk if they are in fact cracked. They're not. The machine shopcan repair thembymachining out and replacing thevalve seats in thecracked combustion chamber. They should be able to do a crack repair as well as a complete rebuild of the heads (repace factory valve guides with bronze guides, 3-angle valve cut, re-seat factory valves, new factory valve seals in conjuntion with umbrella style valve seals, andall newlt-1 valve springs and retainers installed) for about $250
Another question...what are the casting numbers on the heads? The number generally can be found in the "floor" of the oil drain galley for the rocker arms. should be a 6 or 8 digit number. the last three numbers are the casting number on your heads. This will tell you alot about what kind of heads you have as far as what they came off of and what performance they are capable of.
If you want more detailed information, I'll be more than happy to help. You can email me any info you have as well as any numbers and their locations as well as any casting marks you find on the heads. My email adress is Cobra454x@aol.com
just be sure to make the subject heading "400 heads" so I know what it is and i dont accidentally delete it.
Look forward to hearing from you. What are you building the engine to do? I'll be glad to point you in the right direction....