The problem with any era, for American passenger cars, is that most manufacturers make a model for a limited time, and then the model is discontinued. Your best bet for the most variety is to do your shopping for 1/87 scale vehicles on E-bay. You may also find some at Truckstopmodels.com and in the Walthers catalog. Here are some you can be looking for (most are discontinued).
Atlas:
1978-79 Ford Fairmont 4 door sedans and station wagons
MainStreet (also listed as Herpa but not sure if ever released under the Herpa name):
1970 Ford Custom 300 series (Galaxie) 4 door sedan and police car, 1968 Plymouth Fury 4 door sedan and police car
Fresh Cherries (MotorMax):
1979 Ford Escort, 1982 Ford Tempo, 1971 Mustang, 1971 Maverick, Pinto wagon, Mercury Bobcat hatchback, AMC Hornet, AMC Gremlin, Chevy Vega, early 70's Ford van, 1977 Dodge van, 70's Honda Civic and Accord etc
ReelRides (Motormax):
1972 Chevy pickup, 79 Cadillac Eldorado, mid 70's Pontiac Trans Am, 1967 Plymouth GTX convertible, 1970 Pontiac GTO
Classic Metalworks (Mini Metals):
1978 Chevy Impala 4 door sedan, police car, fire chief car and taxi, 1970 Chevrolet Chevelle SS, 1966-67 Dodge Charger, 1967 Ford Custom 300 (Galaxie) 4 door sedan, police, taxi, and fire chief, 1961 Chevrolet Impala coupe
Herpa:
1979-mid 80's Mustang hatchback, 1979-82 Buick Grand National, 1963 Chevy Corvette coupe, Ford 427 Cobra roadster, 1982 Camaro Z-28, 1969 -70 Boss Mustang, mid 80's Thunderbird, mid 80's Corvette, and mid 80's Pontiac Grand Prix (all except the 1979 Mustang were also sold under the Monogram Mini Exacts name)
Busch:
1974 Pontiac Firebird, 1965 Ford Mustang convertible and coupe, 1966 Cadillac ambulance, hearse and limo,mid 70's Dodge Monaco and Plymouth Fury 4 door sedans and police cars, 1950-1968 Dodge Power Wagon pickup (All Busch models may still be available)
Brekina:
1964-1970 Dodge A-100 vans and pickups, 1970 Chevrolet Camaro, 1968-1972 Chevrolet Corvette convertible and coupe, 1972 Toyota Celica, VW Bug, VW bus, VW Squareback
(Brekina models are currently available)
Neo:
1979 Pontiac Trans Am, Jeep Grand Cherokee Wagoneer woody, 1976 Cadillac Coupe DeVille, 1974 Buick LeSabre coupe, 1967-72 Ford pickup, 1969 Buick Riviera, 1986 Chevrolet Caprice sedan, 1966 Cadillac ambulance (Neo models are currently available but extremely high detail makes them very expensive)
Roco:
Jeep CJ-5 Renegade, 1977 Dodge pickup truck
Trident:
early 80's Chevrolet pickups vans and SUVs that could pass for mid 70's models
Model Power:
1969 Camaro convertible, 1969 Olds 4-4-2, 1968 Chevy El Camino, 1970 Plymouth Cuda coupe, 1963 Thunderbird (some of these will be added to the Schuco product line)
Hot Wheels (1/87 scale series):
1970 Dodge Challenger, 1970 Plymouth Superbird (with a wheel swap and a little silver paint to detail these discontinued models look good)
Ricko:
1971 Plymouth Cuda convertible, 1963 Lincoln Continental convertible
Bub:
1966 Shelby Mustang fastback, 1971 Datsun 240-Z (currently available on Amazon)
Wiking:
1964 Chevrolet Malibu hardtop, 1965 Ford Mustang convertible and coupe , VW Beetle (Mustangs currently available)
Walthers:
1966 Ford pickup, 1967 Ford Mustang fastback
Tyco:
1968 Chevy Camaro RS, 1968 AMC AMX coupe (cheap plastic but with interiors and chassis borrowed from other models they can look great)
Road Ragers (1/87 scale series):
1962 Chrysler (Plymouth) Valiant 4 door sedan, 1960 Ford Falcon 4 door sedan, 1961-63 Ford Falcon wagon, 1966-67 Ford Falcon 4 door sedan (Australian product line currently available on Amazon but the steering wheels are on the wrong side but otherwise look American)
Revell:
1961 Chrysler convertible, 1961 Imperial coupe, 1961 Dodge Polara and Dart coupes, 1961 Plymouth Savoy wagon, 1961 Dodge Lancer 4 door sedan, 1961 Plymouth Valiant wagon
Hope this helps get you started in your search. While there may not be many currently available, there were a variety of 1/87 scale vehicles made for the era.