The first problem that I would address with your prakes is the path of the brake line to the rear. That is too close to the exhaust for me. You can easily boil the brake fluid and cause several brake problems.

Correcting this in itself may be enough to satisfy your braking needs. If not, then remove the master cylinder and find out what size piston is in there.

On another note, I have the same finned drums and have plenty of stopping power-on the original 4600 pound car.