So you have used the same calipers front and rear, right?
Often, rear calipers have smaller pistons, so they will exert less clamping pressure on the rotor than in front, all other factors being equal.
I think that an aftermarket adjustable proportioning valve is a great way to reduce the pressure to the rear calipers so they do not lock-up prematurely. I do not think a factory (non-adjustable) valve will work well.
As for MC's...... quite a few Lincolns in the late'70's/early '80's used 4-wheel disc systems, including the MKV, Versailles and the Fox-body Continentals (82-87). Hopefully, from those you should find something that fits.
Good luck
