In either case, it won't be a match depending on how old the sign was because of the lumen maintenance over time.
Be aware some tube bender won't just weld on new glass to the old because a lot of tube benders don't want to mess with the process of washing and cleaning out the glass before welding and processing. The best way/right way would be to do the entire lamp. But not every customer is up for paying for that, but let them know that it will not match.
+1 on the black light to find the best glass mfg