pretty much all the casting numbers are visible with the engine in the car.

1. Stamping: passenger side front of block just below cylinder head. Two letter code, 1/4" tall.

2. Casting: passenger side rear of block just below cylinder head. Six or seven digit number, 3/8" tall.

3. Casting: both sides, center exhaust runners. Usually one digit on each runner. If it starts with 4, 5 or 6 on the left runner, there will be an additional 1/4" tall stamping on the leftmost boss on the outer side of the cylinder head.

4. Casting: adjacent to distributor base. Four character alphanumeric.

If you have high compression 400 cylinder heads on a 350, that can lower compression since the cylinder displacment is smaller.