93 LWB: replaced ABS setup with regular RR servo/mc. Kept the two rear lines and T-ed them in the engine bay. Also kept proportioning valve (same as in non-ABS RR I believe) and pedal (the Disco one mentioned in doing this conversion measured the same).

Braking is good but not great in that I cannot get it to lock at any speed. Is this normal?

I'm also hearing a woosh as I press the brake pedal and it doesn't return back up as quickly as I think it should.

Sorry, can´t be of any real help in your "woosh" problem. Don´t really know the ABS-system.

Easiest way to improve the braking power definately is to take Callipers from the 110" Front Axle. They fit perfectly (you need them with metric mounting holes and probably vented). Clamping power is 25% improved over the RRC item. Doing the rear calliper-swap is possible but not that worthwile (have that in my RRC, too)(the 90" rear callipers are of no help because the are identical in the relevant sizes to the RRC one, if you insist on - then take the 110" rear calliper; but they leave little space between them and the rotor. A spacer/washer is needed in the rear when mounting the calliper). No need to change the rotors.
The 110" callipers are single-line fed. But it seems like you´ll have no problems in modifiying the lines or blanking them with bleed nipples where necessary.
The difference in braking comfort is superb. More than compensates for the 32" tires that my RRC wears.

What do you have - SWB or LWB? Can you lock them up? If a non-ABS RRC is not supposed to lock up then I'd consider mine OK. But every other car I've driven was capable of lockup.

It sound like they need to be bled again.

-Jeff

Mine does the same thing. I am running a jeep booster with a corvette 1 1/8” mc (more than enough pressure to stop the truck). I have used multiple different proportioning and residual pressure set ups and still won’t lock them up. I bleed the thing to death and am confident that all the air is out of the system. I converted the 2 circuit abs calipers to a single circuit set up. I think the conversion between dual a single circuit somehow causes the truck not to lock up.