Don't know if they're still available new, or not. Can probably locate a used one on one of the Pontiac forums, like PY.

Here's one on the Summit site.

https://www.summitracing.com/parts/ofy-5611

You'll probably have to use a side inlet Q-jet on the rear. Probably best to use a side inlet model for both. Says you can't use a big cap HEI dist.

Assume you'll have to rig your own linkage. Will probably need to be pretty accurate, for smooth street operation, since it appears as tho the front carb feeds the front 4 cyls, & the rear carb feeds the rear 4 cyls. But hey, Pontiacs worked with Tri-Power & 2 AFB's, so why not with 2 Q-jets.

Not sure if you could use a dual square bore carb intake, and use Q-jets on it, with adapters, or not. Probably could, but might take a little massaging. The reason I mention this is because I figure you're more likely to find a good used one, for a reasonable price. Assume there have been lots more of those sold, thru the years. Saw a '68 GTO with one of these, with AFB type carbs, a few years ago @ our local car show. Looked good.

https://butlerperformance.com/i-316...ow-rise-2x4-intake-manifold-1965-up.html

https://www.summitracing.com/parts/ofy-5499/make/pontiac

There were also some reproductions of the Edelbrock P-65 intake. Have read on a forum that this one makes more power than the others. Don't have any idea which of these intakes you might be able to find.

https://www.coasthigh.com/product-p/ede-5450.htm

https://ampperformance.com/i-9550385-edelbrock-5450-pontiac-p-65-dual-quad-intake-manifold.html

Here's one that sold recently.

https://forums.maxperformanceinc.com/forums/showthread.php?p=6280905