I intend to give this a full writeup but here are the broad strokes. The idea is to use an available evaporator (the one from the rear A/C of a Toyota Land Cruiser -- $60) and an available expansion valve (the one from the rear A/C of a Dodge Durango -- $30) and have them more or less drop into our E3/E9/E12/E21 evaporator box. The way the project is winding up it looks like the box needs to be slightly modified for clearance and the tubes out of the expansion valve would be custom.
I did a test today. I tied a trash bag on my stock evaporator box that I just took out of my car and an identical bag on my junk yard project box and turned both blowers on to see which bag filled up faster (proving that one or the other had better flow). It was a tie every time, so that's a good sign. As for cooling advantage I feel like that could only be measured side by side with a car with a stock A/C on a hot day, maybe at SoCal Vintage.