Are they all the same block and will the crank from each one fit onto another?

I ask this because I have a harmonic balancer from a 86 305 and am trying to see if it will fit on a 68 327 that I am putting in my car.

 

I believe the answer to your real question is it will "fit", however the timing mark will not match up properly to the '68 front cover.

The answer to the first question is they are different "blocks" from a casting standpoint, but the same externally and you can swap cranks around. Putting the 305 or 350 crank into the 327 block will make it a 350. 327's and 350's could be made from the same casting.

 

The short answer is yes; the balancer will fit and work. But the timing mark might not line up with the tab on that engine, depending on what timing cover you use. If you use the 305 timing cover, it will; if you use the 327 one, it won't; if you use aftermarket, who knows?

The 305 and 350 have the same stroke, 3.48"; and their most common crankshafts are made from the same casting, part # ends in 442. The 305 bore is 3.736", the 350 bore is 4.000". The 327 and 350 have the same bore, 4.000". Many 327s and 350s were made from the same block castings as there is no difference between them. The 327 stroke is 3.25".

If the 327 block is 68 up, then the blocks and cranks will all interchange. If the 327 block is 67 back, then its bottom end internals are not interchangeabe with the later model engines.