You'll have to check if the A72 can accept a rear axle from another Mitsubishi with rear disks. I'm not knowledgeable on these conversions, but from my research on vintage Corollas, it can be done. (most KE70s and KE20s that have been modded for track use have AE86 axles with rear disc brakes).
Before considering a rear-disk conversion, have you tried the usual quality brake pad replacements, service of the calipers, booster etc? From what I've read, 80% of the stopping power is generated from the front disks so good front disks will give you the stopping power you need.