The $620 SSBC kit does exactly that. 73mm caliper,
upgraded master cylinder, performance pads and steel braided lines.
1987-93 Mustang 5.0L Front 4-Lug 11" Big Brake Kit by SSBC at LRS - Same Day Shipping!
I'd consider that a pretty good bang for the buck, wouldn't you?
I certainly wouldn't consider it bang for your buck.
Other than the MC the rest of those parts are dirt cheap anywhere.
I know you are a pretty knowledgeable guy, so do some reading on what makes brakes better.
Rotor size for more area to dissipate heat, those are no bigger.
Clamping force, Really no more clamping force there.
Spreading out the clamping effect of the pads on the rotor, well, 60 to 73mm isn't much of change, that's about .5 inches.
Now compare it to the dual piston pbr's, which spread the clamping power over the entire pad evenly with 2 pistons over a big ass 13 inch rotor.
Rather than buy that kit you linked us to, just buy the steel lines and sleeves from maximum motorsports saving you time and money.
Then do the rear discs.
I'm not telling you to go 5 lug, i'm telling you not to spending too much money on 4 lug brakes.