Erik,
To give you an idea, this is what I did to my '73 frame. As crossy said, this is difference. I used a small cut-off wheel. You WILL need to get the '81-87 brakects for the support. Just us the frame bracket as a guide.
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Chris Lucas
1973 Chevy C-10
1975 Chevy Silverado
Captkaos Customs
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Chris Lucas
1973 Chevy C-10
1978 GMC Jimmy (2WD) - SOLD
1987 R10 twin turbo LS
1991 R3500 SRW CrewCab
1985 K5 diesel swapped project
1989 K5 2WD conversion w/ Vette susp Project
Captkaos Customs
73-87chevytrucks.com