so vintage and classic are not exactly replacing the box with a b-body specfic box.

They basically have a box they setup to "fit" in the same space.
they don't poke out past the firewall like an original box, so they have a lot of leeway on how to mount it up.

If you are just looking for aftermarket a/c, you can still do it, but you will have to do some minor fab for it to work.

for instance, when I did my retro fit with classic, they did not make one for the early b bodies.

so I purchased their universal kit for hot rods.
I gave them my grill opening size and they figured out the biggest condensor I could fit in the opening.

I then decided on the electronic box instead of the vacuum operated box. It used 3 knobs instead of the original heater switches.
was a cleaner install because I wasn't trying to get any of the old stuff to work.

It has been 5 years I guess, and the only problem I have had is with the heater valve. it doesn't move when the valve is bottomed out open or closed, but can do it when it is in the half way position.

Anyway, to get that to fit, I made a plate that had bolts sticking through that matched the firewall mounting holes for the original heater core. had feets weld the bolts in place.
then bolted their box to that plate.
no new cutting except for the a/c lines. BUT you could get one of their cover plates for the ebodies and run your lines through the big hole for the blower motor instead.

I did mount it a bit high, so couldn't get the defrost vent hoses hooked up, but not really a big deal.

you get lines cut and crimped local by a hose shop, and then get the system charged up and you are done.