340 valve sizes 1.818" intakes and 1.380" exhausts. I believe you can fit the 1.425" odd-fire 231 V6 exhausts in the 340 heads. I have a set of heads from a '65 300 at the machine shop that are being fit with the V6 exhausts. The valves alone won't make a big difference, but the porting and bowl work will.
Thanks for your reply but I don't think 65 300 heads are like 66-67 340 heads. The 65 300 head is similar to 215 I think. Someone else can correct me if I am wrong.
Hi Julian; You're close. The '64 aluminum 300 heads were slightly enlarged versions of the 1961-63 heads on the 215. The valves in the '64 engine are larger than those in the 215 however. In '65 Buick made the first of its cast iron heads for the 300. The '65 heads have slightly smaller combustion chambers than the '66-'67 heads, which helps to produce the 10.25 compression ratio in the 250hp engine. The cast iron heads are relatively the same design as those on the 225-231 odd-fire V6 engines. However, the V6 had smaller intakes and larger exhausts than the iron 300. The '75-'77 valve stem diameter and keeper grooves are the same as the '65 300. Mark
