The idea of the tbody spacer goes back to carburetted engines, when smoothing out the airflow path helped to keep the fuel from pooling in the "corners" when it turned into the the intake runners and into the cylinder head from there. It's the same sort of thing that allows the HO head to make more power - the air flows in more efficiently than in the Renix head.
With "dry" manifold setups (MPFI/TPI,) there is no fuel atomised in the intake airstream to pool anywhere. There is a small benefit, as I understand it, to be gained from the increase in intake volume, but I don't recall it being enough to be worth doing the mod. If you are thinking of some serious power mods, consider a "big valve" head swap from Clifford or Accurate Power, and use a late (HO) head and intake setup to do the swap. Couple that with Mustang SVO injectors and you can really start making some progress. You can really get somewhere if you put a stroked short block underneath that head...
There are a lot of power discussions to be followed on the "strokers" board (groups.yahoo.com/group/strokers) with a larger exposure to the Jeep community (sorry, but we're mainly XJ/MJ here, and strokers is pretty much the entire AMC/Jeep inline six community...)
How much power are you trying to gain?
5-90