My 72 El Camino was converted using parts from an 89 Camaro. The CCOT evaporators will fit in the stock box and gives a place to install the orifice tube. Not sure what the pipes on the truck evaporator look like, if oriented correctly it will be a bolt in. If not look into buying an evaporator from a 77 - 79 Nova.
When converting the Nova the factory cycling switch was removed and replaced with a multi function cycling switch. The orifice tube replaced with one for 134A. The evaporator and condenser were flushed, the accumulator, hoses and compressor were all new.
Photos of the El Camino and the Nova, both use 134A and work well.