Warn Enforcer bumper works on '81-'87 pickups, but not for '73-'80? Whats the deal?

['65 Super Sport 327 700R4]

I want to get a winch bumper for my '74 GMC pickup, but the bumper I want only works for '81-'87 GM's. Why cant I use this bumper on my GMC? If I did use this bumper would it look like its made for my rig? Any of you guys know where I can get any aftermarket front bumper (new or used), that will hold a winch, for my truck?

Thanks,
Erik

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'29 Chrysler model 65 all original, 2nd owner ***FOR SALE***
'63 Impala SS 283, 3-on-the tree, flowmaster 2 1/4" dual exhaust, performer intake, edelbrock 600 carb, chrome 15x8 rally's
'65 Impala SS, built 332, 700R4, 3.73, 2 1/2" mandral bent duals w/H pipe, Flowmaster 40-series deltaflows, MSD Mallory, too many modifications to list
'72 Chevy Suburban 4x4, NP205, TH350, balanced 357 (.040 350), 268 high energy cam, 9.2:1 comp, 1.94/1.50 heads (ENGINE BUILT BY THE GREAT RAYZOR or if you know him RAY, WHO POSTS ON THIS BOARD AND CHEVYTALK), 3/4 ton leaf springs, 4" lift, 35/12.5/16.5 Grabber MT's, Rancho shocks, Rancho dual steering stabalizer, American Eagle 143's 16.5x9.75 6 lug, 4.10's, 12 bolt rear limited slip, Dana 44 front limited slip. Carter 600, performer intake, headers and flowmaster exhaust system very soon.
'95 Impala SS Flowmaster cat-back, K&N, 285/40/17 Firehawks, Dark cherry metallic, 27,000 miles, extreamly fun vehicle
'95 Chevy K2500, ex-cab, long bed, 350, AT, Flowmaster cat-back, 265/75/16 Mud Kings, 16x7.5 Centerline HT cast
wheels, Leer high rise canope, bed liner w/ mat, loaded.**FOR SALE**
***I am looking for a '67-72 pickup, suburban or blazer in the Portland area. Body doesn't really matter but it needs to be in great mechanical condition. $500-$1500 range please. ***