Need advice on Brake line parts and configuration
Since your going to modify the brakes, and your there, hell yeah I think you should do it!
With a 73+ Mopar factory prop valve rear, its recommended you pop off that bottom nut and take the guts out first, then run the adjustable valve sometime after... I hear you can get weird cross talk between the factory and aftermarket valves if you don't.
I've seen a bunch of folks keep it simple and just tee the front brakes and run an adjustable valve to the back, removing the factory valve altogether but you don't get the fancy low brake light..
Since you have everything, you can either try and muscle off that nut off, or do what I'm going to do shortly; work around it: just pull the inlet, and outlet from the factory valve and put the adjustable valve in there with a length of pre-flared brake line found at your local parts store to make up the difference. Bent to perfection by yours truly. Plugging the factory valve. Not as clean, but an option.