I have a list of casting numbers that will tell you if it is a 400 or a 350. What is the casting number of your block? Look at the front, if you can, just near where the camshaft inserts (under the timing cover).
Many (but not all) have three freeze plugs per side on the block versus two for the 350. I've had three 400's and none of them had three freeze plugs per side (although there was a raised casting area for a third one).
A 400 never came stock in that car so unless it was swapped it's not a 400. All the standard 350's and 400's have the rear mounted distributor (except late-model LT1's and LS-1's).
another way to tell is the harmonic balancer will be taller and almost a 1/4 inch will be milled of almost half way around the balancer and another way is on the side of the block the cylinders will be a little swollen if it is a 400 as the later 400's only had 2 freeze plugs
Also, if you have the engine out or party disassembled enough to see the casting numbers, then you may likely have a cylinder head off too. If so, look for the presence of steamholes between the cylinders as opposed to a water jacket. The cylinders will be visibly closer together too.
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