My evaporator housing is also cracked. It has three holes in it. I too initially thought it had been damaged when the BB engine had been installed by a previous owner. I've come to the conclusion that holes just naturally occurred, being caused by the fiberglass housing being heat damaged by the exhaust manifold. With a BB engine, there's only an inch or two clearance between the exhaust manifold and the evaporator housing.
The factory AC appears to be grossly inefficient. The evaporator housing is a little over 1/8 inch thick. On one side of the housing there's air in the mid 30's (F) blowing out of the evaporator, and on the other side only an inch or so air gap separates it from the 1000 degree F, or so, exhaust manifold. Also, on a BB, the end of the compressor is mounted to the exhaust manifold!!! I've decided to replace the factory AC with an aftermarket unit, probably Vintage Air. With the Vintage Air unit, the evaporator assembly is inside the passenger compartment..away from engine heat. Also, with Vintage Air, the entire heater/evaporator housing disappears from the engine compartment freeing up a lot of space. So no potential problem with a BB and headers. In some way, I'd like to keep the factory AC appearance, but I like the extra engine compartment room I'll be getting.
A few weeks ago on one of the corvette forums, someone was selling a completely restored evaporator assembly for $399. This included the POA valve, etc. It had been restored by Classic Air in Florida. Forget which forum I saw the unit, maybe this forum or CF. Classic Air recommends opening up the POA, the expansion valve, etc and cleaning them before re-installing. They will also recalibrate for R134.
I've bought used parts from Paragon and J&D Corvette (Bellflower Ca) also, from salvage yards advertised in the NCRS Driveline magazine.