$600 rb20's? hmm
the 100k servicing needs to be done anyhow.. whether its an rb20, rb25, rb26.
rb25 box, well can be left for a while with rb20 box (like mine) or buy another. An rb20 with similar power still requires an rb25 box at some point anyhow, with the tailshaft mods, etc.
Wiring mods? quite easy.. if you have the full rb25 engine loom you can work it out, direct to rb25 ecu. I'm not an auto elec, nor a mechanic, nor ever rewired a conversion. Factory engine+ factory ecu = engineerable to get it plated. Plus you save $400 on the PFC as opposed to getting the AP version (that's near a full tune).. readily available, and under the $1k mark. Plugs right in.
I've got the spreadsheet in front of me with the numbers, roy - it's not expensive. $2.5k for .. manual conversion + rb25 - sale of old rb20 parts. Just shop smart. My engine may not last forever, like any second hand engine, but there is always the option of an rb30det, or rebuilt rb25, or simply another rb25 to slot in. The framework is in place.
But each to their own