I did a vintage air swap myself a few years ago. Used the compac unit. Aftermarket takes some fabrication work such as making a firewall block off plate , hanging the unit,severely narrowing or removing glove box compartment,adapting ducts,New custom hoses, mounting sanden compressor,fitting aftermarket condensor, control unit mount ,etc.The aftermarket units have the advantages of more underhood room, cleaner look ,more efficient output per sizing of components, servo motors instead of vacuum components, And everything being brand new.
As far as ac shops go there are only a few that do the complete swap type work.Here in jacksonville,fl , King auto air & Catlin's on main street are the main places that do swaps.
As Dave V was hinting on, if you have a factory air car, it could be cheaper to go back factory air if you prefer, I just plain was tired of this & that breaking here & there & wanted the cleaner look around my LS1.Also just to mention per other forum discussions, many southern hot rodders prefer the Classic Auto Air brand over vintage for having a little larger components for our hot enviroments. The vintage air compac does wotk better than the dirty coiled large factory unit though.