Replace the condenser, I use the 34" wide suburban/tahoe with rear air replacement micro tube parallel flow on all of these trucks. If the orifice is at the condenser outlet, use the Cadillac Yellow 0.062 orifice. It is also called out for on the last of the SUVs like a 1999 Suburban. If the orifice is in-line near the evaporator core I use the Ford Red 0.062". I do not see why adapter brackets would not mount a Sanden to any R4 bracket. Its just a universal bracket that bolts to the R4 bracket pattern and the Sanden ears. I swapped a Sanden in place of a V-belt R4 about 5-6 years ago on an Oldsmobile 350 diesel powered Caprice wagon. My personal preference would be to adapt a ~2003 S10 2.2L V7 compressor as WildBlue19 has done and now has two of them on his Suburban or swap to Vortec brackets to run a HT6 replacement. WildBlue19 could probably make some money having brackets water jetted out to his design and sell them. I will never mess with the unreliable R4 again in anything and the 87 G20 van is proof of that. I will not even touch a system if someone wants to keep the R4. The R4 Scroll is not a good idea if the truck will idle around down in Houston much, it is a lot like the Sanden, inadequate at lower rpm. I get my bulk a/c hose and fittings from Cold Hose and have an inexpensive amazon sourced hydraulic ac hose crimper that works easily and quickly.
Kwik Performance sells a set of adapter brackets that bolt a Vortec 4.3L-5.7L accessory bracket setup directly to a 454 as well. I used a set of those on a 1971 C20 I helped my buddy setup that had a 402 in it. Could use the 10 cylinder upgrade HT6 I have on my 97 Express van that way. 454 vortec accessory brackets are also fairly common to find. You can re-use your TBI power steering pump. Also a good way to be able to run a much more reliable LS Tahoe AD-244 145 amp alternator. The AD244s can put out nearly 100 amps at hot idle and are a far more reliable design overall than anything that came on these trucks.
I would replace the hoses, condenser, accumulator, upgrade to a better compressor setup, at a minimum flush the evaporator core and run the 0.062" orifice. Proven formula for me to get ice cold air even with R134a on a old R4 system. Most of these trucks are missing the accumulator blanket and the suction hose insulation. GM sells the accumulator blanket for the newer Express vans reasonably as an AC Delco part number, it fits all the older accumulators very well.