My first 2bbl to 4bbl swap.. scratching my head..

I hope you don't have A/C, because the bracketry can easily drive you insane when you go with an aftermarket intake.

Definitely start out by making sure the ignition is up to speed, and timing is set correctly. Since you have a hotter cam, try a little more advance and see what that does for you.

Down the road, consider an electronic conversion kit to eliminate points. They're $200 from Mancini. It's not "a must", just something to consider.

Assuming you do go with a four barrel... the 318 carb IS pretty tiny...

I don't know what your budget is, but I like Lokar products. It replaces the kickdown rod with a cable, and all of it mounts to the carb, so no brackets on the manifold bolts. Plus, being cables, there are no alignment issues when you change manifolds. You're talking $67 for the trans kickdown, $43 for the throttle cable, and $26 for the bracket. Check this out: http://static.summitracing.com/global/images/instructions/lok-kd-2727htkd-2904htkd-2518htc.pdf

The alternative is to cut and weld something up, or try to find factory or reproduction parts that will fit. Repro parts ain't cheap either!