If you go with the R151, you will have to use it's t-case, or rebuild the one you have due to the trans being 23 spline and your current t-case being 21 spine. If you go with the R150, you will have to buy an adapter plate to bolt a 23 spline t-case to the R150 trans, or use the the R150's chain driven t-case.
As far as strength goes, the R151 was behind a 134hp/178trq engine, and the R150 was 150hp/180trq engine w/the 3.0 and the 190hp/220trq of the 3.4(254/270 w/the TRD supercharger)
If the R series transmissions were all built the same, then either one should live behind the 350, guys in the Supra community can run 400+rwhp w/no premature failures of the R154 behind a built 7mgte or 2jzgte. You might give Marlin a call/email, and see what gears can be used in the R151 and R150, if they are interchangeable w/the R154, the 154 has a 0.753 5th gear.
w
