I believe the 77 PX10 still had Simplex derailleurs, and if so you need to stick with it unless you are willing to permanently modify the derailleur hanger or use a bolt on claw type hanger. I looked at doing this to my 72 PX10 to modernize it but I did not want to bastardize the frame and change the chain line for a wider hub. It also becomes expensive as you change wheels, drive train etc.
I would buy better tires on line or change out the rime as I mentioned in post
#7 above.