Well all 427's have the same 4.23 cyl bore both early and late. But it's good the block is still std bore. Hopefully the oversize rings do the trick. Most 58-66 heads were lowrisers. Exceptions are the 63-64 high riser and 65 medium riser. But I can assure you the C2AE-6090-B 406 heads are lowriser. But measure the intake ports 2.34 x 1.34. Exhaust ports are 1.84 x 1.28.