Pontiac V-8 architecture never fundamentally changed from '55-81, but external appearance and accessories did.
For a 1968-1970-something my vote would be for the following, knowing some are a touch oversize for the task, but relatively few individuals would get out the micrometer to know for sure.
1. Revell 1969 GTO Judge - oversize in 1/24, but not enough to bark at. Correct accessories, and kits were going begging at $12 at the last show I attended.
2. Revell 1970 Trans Am - available now, kinda hate to blow one up, but it is right on the money. You will need an air cleaner, see above.
3. Revell 1966 GTO - best in 1/25 by a smidge, needs a single 4V, carb and air cleaner. Super nice engraving
4. PL 1964 GTO - don't let the simplified assembly fool you, very well rendered just needs the single 4bbl and air cleaner, valve covers maybe if you are super picky.
5. Lincoln Mint '69 Judge - starts life as a diecast, so there's a strike but I recall the engine being pretty good. Also a very affordable item, when you find it.
6. AMT '65 Bonneville / GP / 2+2 - engraving is pretty '60s on it blunt and soft. But it has presence when the hood goes up.
7. MPC '70 Bonneville is cut from the same master as the GTO, but the Bonneville tool has not been flogged nearly as hard as the GTO.
Parts pack engines are best to avoid, honestly for a later model build, unless you are trying to make the engine look like 1963. Revell 421 has the advantage for a '56 pickup build, though, the stock front cover and water pump, tri power, etc are bolt-ons to the clone in the kit and get you a good '61-4.