350 heads on a 307
#2
Heads from WHICH 350? The 1977-up motors used 1/2" head bolts and the heads will bolt directly on. The 1968-76 motors used 7/16" head bolts so the holes in the heads need to be opened up. Of course, you also need to verify the combustion chamber size. The 307s used heads with 64-67 cc chambers, same as the early 350 heads. The No. 8 heads have 80cc chambers that will drop CR so low that you can run kerosene and not detonate. What size valves are in those 350 heads? The 350 has a 4.057" bore. The 307 has a 3.800" bore. And finally, which year 307 exactly? The ones with 5A heads will have compatible intake and exhaust manifolds. The ones with 7A heads will not.
#3
If you have 5A heads, don’t bother, not enough gain. The only heads I would mess with, and even this is a stretch, are #3 and#4 heads from a 330. Smaller combustion chambers but You lose .024” from the thicker replacement head gasket. Still need to drill the head bolt holes to 17/32.
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