I'd like to throw my two cents in.
RB20 box is shorter than RB25 hence the different tailshaft lenghts. But since you have a welder you can fabricate/modify the tailshaft using RB20 yoke.
Engine mounts, should use the R33 ones I suppose. In R31s you use the R31 mounts for RB20/RB25 conversions or the engine sits weird.
Best is to dummy mount the engine and gearbox and start measuring.
Also, since you can weld, make a Gibson-style exhaust manifold.