This got me thinking. I have a 68 bird with a 69 400 engine. Its a YD code, which I looked up to find its a 290 HP with its original #46 heads. I was told the original block cracked and this one was swapped in, but the original 1968 #16 heads were reused. I am assuming that changes the HP rating, a block is a block right? If the bore and stroke are the same, its the heads that change the HP?
You're right. The block and rotating assembly are not what matters in your case. It's the heads, cam, intake, and carb.
Depending on the year of the block, the cranks did get thinner counter weights. That happened in the mid '70's. Top ends can be put on most anything.
