Pontiac V-8's, from 326 to 455 are all the same size externally- where they differ is in the main journal diameter. The 326, 350P, 389, and 400 engines all have 3.00" main journals- 421, 428, and 455 have 3.25" main journals. Each engine is also a unique casting not just simply bored out, so you can't just bore out a 326 to 350P or 350P to 400. The only two engines that make somewhat of an exception are the 389 and 400- the 389 has a 4.0625" bore and the 400 a 4.12" bore, so a 389 can be bored .060" and use standard 400 pistons. All 326- 455 Pontiac V-8's also use the same 6.625" long rod. The 326,350P, 389, and 400 also share a 3.75" crank- PMD did put different no. 3.75" cranks in each of these four engines, but they do interchange.
Since the 326-455 family of Pontiac engines are all basically the same size, engine accessories will interchange, but car has to be taken since pulley alignment will be different from the 326, 389, 421 engines to the later 350P, 400, 428, and 455 engines. Heads, intakes, oil pans, flywheels and starters will also interchange and fit each other, but care must be taken not to use the wrong head on too small or too large an engine. Pontiac V-8's use flattop pistons, so compression ratio is set by the combustion chamber on the cylinder heads- use large chamber 455 heads on a 350P and you could get a horrible 6.42:1 SCR, or putting 326 heads on a 455 could yield a piston-hammering 12.9:1 SCR. Large valve heads can also have valve-to-bore interference on small bore engines like the 326 and 350P- some of these blocks are chamfered to allow larger valve heads to be used.
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