stock heads are fine and with a performance valve job will support 400 hp if you can find a decent set not requiring a lot of work.
For the street build it to about 340 - 350 hp and 400 ft. lb.That is plenty in an A body.
This is true, 340/360 with 284 degree .500 lift cam, 2.02 x,j,587 etc..cheapo headers, hi rise dual plane, 750 carb, 9.5 -10.1comp will make about/around 400 hp
