sending unit

[va_tlc]

my gas gauge doesnt work, i pulled the seat after grounding out the gauge on back of dash it worked, well i check the wire that runs to the sending unit and yep have juice there too, pulled aanother sender from a parts truck and hooked up b4 i droped it in the tank it worked while moving the float up and down ohh installed a new float also. well after i put it into the tank it wont work,,, forgot to mention while out of tank i grounded it against the tank, question is should i run a ground wire? or is it suppose to ground out when putting the metal hold down ring on? hope this is clear enuff for yall to understand thanks in advance, with 4.55 gears i just gotta know how much gas i have, or better terms dont have ohh and i looked for new sending units and it list 3 diff ones my cab is a 72 auto originally 6 cly 2wd... now its a 72 3/4 4 speed v8