Depending on what you are converting from..
Things to consider are:
barrel chamber
ejector (right length)
extractor (aftecs work well, otherwise adjusted for the case rim)
firing pin (fitted to avoid primer flow)
firing pin stop (oversized to keep the extractor from moving)
breech face width (not too big, not too small)
recoil springs (right weight for load..factory or handloads)
Other than that...I'm sure there are other things to throw i here.