You'll need to make a jig to support the brass side plates at the proper angle. You'll also need to know the dimensions of the cooling area of the heater core your going to use. The Chevy Chevette heater core I used has a cooling area measuring 6”x 6-1/16”.
I used my table saw and set the blade at a 22-degree angle and the fence at 5-7/8". Using 3/4” plywood I made the first cut. Next I rotated the plywood 180 degrees to cut the other side. Then I moved the fence out to 5-15/16” and cut” the remaining two sides. These measurements may differ from heater core to heater core. Just remember to subtract 1/8" from the overall heater core size when making your jig. You'll need to compensate for the thickest of the shroud side plates.
The plywood jig
I returned the table saw blade back to zero and set the fence to 10” and cut a 3/4” piece of plywood so it was square. You'll need to find the center of each piece of plywood and pre-drill a hole in the center of these pieces. You then take the plywood that has the angle cuts on it and screw it to the 10”x10” piece of plywood.