If you are failing spokes in 50 - 75 miles, something is off here. I'm guessing since it is older bike you have 36 spokes per wheel. Also, you did not mention which side fails, DS or NDS, hopefully spoke length is adjusted to account for the difference here. I don't see why you would need to spend $$ for triple butted spokes, DT or Sapim double butted 14/15/14 should be good in a 36 per wheel set up. Weight, as in yours, might be a consideration also, probably plenty of info on weight, # of spokes, rim, etc out there in internet land to gauge what you need. I also prefer double eyelet rims or at least the washers in not double eyelet. Others may have more info. Just my thoughts for use as deemed appropriate.