It's worth noting that you probably need some major butteflies in the Shuttle program to make this possible--Skylab will re-enter in late 1979 without a reboost, so if Shuttle isn't there to do it, Skylab's coming down. The plan was only viable when it was hoped Shuttle would be ready in '79, but issues with both the SSME and the TPS caused a two-year delay to 1981. Thus, you either need a different Shuttle development program to have Shuttle ready in time, or you need an earlier reboost mission--perhaps one of the proposed Skylab 4 flights, which were proposed to involve reboosting the station.
There was an unmanned version of the Skylab reboost module that would have been perfectly practical, though that gets things well into committing to additional hardware right when the Shuttle program is at its ugliest financially.
You probably also want to look very closely at ASTP if you're thinking about Skylab B. There was some definite talk about using that way, but the logistics weren't great and it was pretty clear ASTP was going to be a one shot from fairly early on. Nonetheless it's probably the best shot of getting the station flown without other major changes; if it docked with a Salyut things get very complicated very quickly, but I could pretty easily see ASTP being conducted as a pair of missions, with Apollo visiting a Salyut and a Soyuz visiting Skylab B (AFAIK the Russians couldn't have made Skylab A's orbit at all). You would probably even have enough delta v to get both stations on a single flight (Salyut 7 to Mir and back was done once, and Apollo had quite a lot of thrust to spare even with the partially fueled Saturn I launched orbital vehicles).
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