Thanks guys for the replies.
The brake system is 100% new and adjusted properly. I spoke to a number of local mustang guys, and with the hand brake, they are problematic at best.
Lots of brakes have busted handles, because you have to reef so hard on the handle to get them to work.
Restomod guys typically use a lever from a modern car and mount it on the driveshaft hump.
I have killed my budget on the engine, so will mess with the p-brake for a while.
The other thing might be to fabricate another lever with a better mechanical advantage.
If I ever get it working, I'll let you know,