AMC offered TH400 in the Qtrac Jeeps behind the 304/360/401. A tranny guy can change the main shaft & tailshaft hsg to make it a normal RWD TH400. Then you get a 9" ford the width that you need & have a driveshaft made to connect the two. This could be done for not alot of $$, just a lil bit of fabbing. The ambassador has leaf springs in the rear, doesn’t it? You could prolly get away with an 8.8" ford from and explorer or something similar. As long as you aint got mega HP, it should hold up well. Maybe even find one with rear discs.
Good luck with it.