I need them to be the right distance apart, both side to side and fore/aft, and I need the top edges to be in the same plane. I decided to try 3D printing these little jigs, which I will position and screw to a piece of plywood.
The plywood board is clamped to the frame of the car. The blue plastic jigs hold the tabs. I've used the transmission subframe to get everything in position, with some clamps and spacers to help align everything.
I will next finish prepping the subframe and the tabs and get it all aligned, then tack it together and test-fit it in the car. I think it should work; then I can finish weld it, bolt it back to the bottom of the transmission, and tack it into the frame.