Re: SBC to BBC conversion '68-'74
A BBC is 2 inches taller (with stock low rise manifolds), and wider and 2.3 inches longer than a SBC (this assumes it is the standard height block, a tall deck truck block makes it nearly 3.4 inches wider than a small block).
You could order A/C with a big block back in 1968-'70 (just not with the 375 horse solid lifter 396 version of it), so I know it can be done with factory parts that are no longer available from your Chevy parts counter. The BBC has different heater cores and heater core covers than a small block uses to move the hose outlets away from those big heads rammed up against the fire wall.
I suspect that the A/C air handler box is unique to the big block as well, and you know it won't be a high demand part for the reproduction companies to mold; but they are making parts for the first gen Camaro and most of the firewall is the same. So see if that part can be modified to fit.