390 w/open chamber heads

I've seen some conversation on this exact subject over in the exhaust tech section--search around in there for more specific info.

They'll work, and you should still reach your goal...think about it for a second. HP 383s were rated in the 60s at 335hp, 440 Magnums at 375hp through manifolds with similar attributes. Granted those were gross hp ratings, but they were also mass produced powerplants...yours should be better since the selection of parts has been more scrupulous, and machining accomplished more carefully...they'll likely fit the bill for your purposes...also, the use of a standard Performer intake, and factory style heads, if anything they'll keep your hp to around that level.

You can port match the openings on cast manifolds if needed. I've seen it done before on cars where a stock look was desired but wanted a small increase in breathing...you won't make any huge gains, but every little bit helps.

You might have to do some grinding on that driver side manifold or possibly alter your steering shaft to allow it to clear the coupler. Those threads are also on here...the grinding on the manifold part is in the exhaust section, and I want to say there are folks that have modded their steering shafts/couplers/columns to allow extra clearance--but I don't recall where I saw them.