its a costly mistake to rebuild a yb with no experience
if your not sure about pistons and bearing sizes then i'd advise leaving it to an expert
if you are sureyou want to try it yourself then take every part to an engineer to get properly measured and checked, it might cost you a couple quid for this but at least then it'll have the correct parts,
dont try rush the build, take twice as long as you think it should take and tripple check everything,
imo a yb is a heavy stressed engine and so not a good jigsaw puzzle to start playing with as everything needs to be perfect or it will go horibly wrong
anyhow i do admire your courage for trying but just be very careful and seek expert advice at every point