Everything 75 - 87 is HEI, 81-up is ESC: Electronic SparK Control - or
Extremely Stupid Crap to those that love to hate it. If you have ESC on there (it will have a computer running to the distributor) you may want to consider GM's solution to the problems associated with the ESC system, eliminate it.
See here for info:
http://www.73-87.com/7387garage/drivetrain/escrepl.htm
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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