The 4-bbl conversion is very straight forward, but you do have a couple of options to be aware of.
The factory cast iron Q-Jet manifolds are actually pretty good. And they're cheap. The manifolds used from 1967 through 1972 use a "divorced choke" system, utilizing a bimetallic spring mounted to the manifold exhaust crossover. In order to use this choke system, you need one of the early-type divorced choke Q-Jet carbs.
After 1973, the manifolds and the carbs used an integral hot air type system, utilizing 2 tubes sticking up out of the manifold crossover, feeding hot air to an integral choke on the carb. These carbs are nice, because you can install a 1980 electric choke into the carb and eliminate the hot air system (allowing you to block the exhaust crossover).
The best factory manifolds are the aluminum Ram Air and HO manifolds. These work great, but are getting pricey.
For a nice weight savings, you can use the Edelbrock Performer series manifolds. The Performer RPM is an outstanding manifold that will actually outperfrom the factory iron unit, and it's available at a modest cost. It is drilled for both the Q-Jet and the Holley/BG bolt patterns, so you can use any carb on it (a very nice benefit). Before running this manifold, be sure to go to the Edelebrock web site and check the height of the manifold against your available hood clearance.
Although your engine is a low-comrpession engine with the small valves, you'll still get a nice performance enhancement with the 4-barrel conversion. Next step after that will be large valves, a cam, and a pair of headers...