1971 340 water pump and crank pulley problem

photos?

Sounds like you are running only WP, Crank and ALT so each could have a single grove pulley on them.
might want to look for a matched set?
In a previous post i measured the CL of the belt on my 67 273 to be about 5" from the machined face of the head not sure if a 340 measures the same.