Fuel sending units

[Lil Greg]

First off hello to everyone ! , and now to the good part questions questions . Ok here we go i'm building a truck from scratch i have a 87 shortbed frame and duel 16 gallon tanks ,75 cab and front sheetmetal , 81 fleetside bed and my my 68 396ci /th400 im yankin out of my 56 gmc and am having problems finding and deciding which sending units to use should i buy ones for a 87 to match the tanks ($400 a piece at dealership) or try to get some for a 75 to match the gauge cluster? and would it hurt to put the 75 units in the 87 tanks?......Whoe ok i'm and my hands are cramping lol......too much typing..............anyone can u please help!