There is a couple of threads on the main board about this. I remember 90's Ford Aerostar van spring is like a 3 inch drop spring.
Hey Kegger... Give Jamco Suspension a call. Jim Genty has great quality suspension stuff just for these cars. I have all their stuff under my 53 and I am very pleased with the results.
One of the members "1955 Ford Wagon" did one of the original posts got His at Advanced Auto for $48 for the pair,tell the counter guy you have a 1990 Ford Aerostar 2 WD. Yours will probably drop about 2" since you have swapped to a SBF which is lighter than the stock motor.