I used Seafoam on my first 85. I pulled the big vacuum line on the end of the plenum and put a hose into the bottle and stuck it into the nipple on the intake. I let it sip it like drinking through a straw. The danged car never did smoke. The exhaust did stink for awhile, though.
I think the recommendations were that part went through the intake, part through the fuel tank and part into the crankcase. Some have reported engine problems after adding it to the crankcase.
In the intake it's supposed to clean the top end of the engine. Buildup in the intake and runners, the valves and combustion chamber.