gas cap ???????????????? help

[72shadetree]

got a locking gas cap that leaks.so i go to napa and look it up in book and get it well it goes on but wont tighten up it starts to but then it slips kinda like a stripped bolt does.so i take the one off the other 72 that fits fine but it does the same thing on this 72.the new cap fits the other 72 fine but not this one.are there different size filler necks used in these trucks.i thought 2 72s would be the same
HELP...........THANKS

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72 chevy lwb 2wd rear leaf truck.long term frame off project.
72 cheyenne 2wd lwb 355 cu. in. auto.daily driver and fixer up.
90 fullsize bronco 351w/at.(no lift yet)
97 lumina v6 auto(wifes)
70 galaxie 351w auto

bassett va about12 miles from martinsville speedway



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