ive used old shells over and again from running engines,not best practise to use unknown bits and pieces though,if they aint scored you can try squeezing them up with plastiguage,if you get a good reading and they aint damaged youll be ok,if its a race/drag engine use new,ive thrashed these little motors for years on old bottom ends with rebored top ends,they handle it,just do it all clean.
don't mix up the caps to rods!run a cap over the rod and youll feel a snag/grab everytime,give them both a few strokes over wet/dry on flat glass and theyll glide with no snags and close up with more precision,polish the shell seating area aswell by hand with say autosol polish,you want perfect shell to backing contact,the heat is transferred squarely then.
which two look polished?its just that its apart now so do it perfectly!