OK, this is a 1" GM MC, with the front brakes coming from the the FRONT half of the MC, unlike the Mopar where the front brakes come from the rear half of the MC. So please confirm that your front brake lines are coming from the 2 front ports.
Do you have the drum size and brake shoe width of the rear brakes?
If you have not re-bench-bled the MC, then IMHO, you have wasted time. How are you doing the bleeding? I ask because there are techniques that don't work well.
Are the front calipers mounted with the bleeders pointing up?
Is this a GM booster?
And sorry to ask so many simple questions but they are being asked for a reason. There are a lot of things can go wrong and they all have to be checked.
