As the old rock 'n roll song goes, "breaking up us hard to do." In my case (pun not intended...or maybe it was) it wasn't so hard to do, but a bitch to UNdo. I'm now leaning toward a Tremec 3550 5-speed overdrive transmission or as an absolutely "quick and dirty" solution, a TH350 automatic (for those not familiar, that's the transmission that was standard fare for just about every smallblock and I6 Chevy from 1969 to 1984) and an "airplane gear" rear end with a 2.42 or 2.73 ratio (I currently run a 3.73 gear.)
That would eliminate the need for overdrive, keep the driveshaft speed down and probably give me the top speed of around 160 that I'm looking for. I can buy a brand-new TH350 for $700, about the same price it would cost to swap the TH200-4R's good guts into the good case. And I can get a used rear end for $100-200. It might be enough to limp it into the 130 club. I dunno about the 150, tho.
Decisions, decisions.