The easy part is measuring BB shell width and required axle lengths.
The hard part is making sure that the cartridge BB available has the thread pattern to match the frame.
easy if you have a confirmed English threaded frame, trickier once you move into French or Italian thread.
I have a late 70s frame with a fairly recent UN52 fitted and all is well, and I always fit cartridge BBs when refurbing frames as they are just about fit and forget.
these TH industries are quit good value:
http://cgi.ebay.co.uk/122mm-Sealed-Shim ... 384wt_1149