Guess I should have looked around the small block area before posting so sorry and I'll just keep looking for another 350 block to find another 350 crank.

 

I think you’ll find that all the small blocks of the era shared the same stroke (3.385”) and journal diameters. I would think the balance would be different on a 403 but that is something I don’t know. Wait for others to reply.

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Every small block Olds motor made from 1964-1990 used the same 3.385" stroke. All gas small block cranks interchange. Obviously the balance may need to be corrected if, say, you try to use a 403 or 307 crank in a 350, but they will physically work.

 

As in 260, 307, 350, and 403.
And the earlier 330 had the same dimensions but in forged steel.

 

Where are you located?