the prop on the rb20 box has a smaller spline than the rb25 so you can either get it cut ,welded then ballanced or change the ends

as for the rb20 and 25 every one has defferent oppinions all depends on what power u want

i no u can use an rb20det loom to run a rb25det with a little wiring, so would say yes to being able to use the rb20 pfc

ask richard aka(bradley motorworks) of the site he would no for sure 100% to deal with