It is not going to matter what block you put in, because they are essentially all the same. The reason some were sslower than others would be due to weight of vehicle, the whole s.d. vs. mass air deal, and stock intake, exaust... I would get an 87-92 for the forged internals, and thats about all I would worry about. All of these engines were virtually the same horsepower. Dont worry about it, if he's got a '94 on hand get it, if not go with an 87-92.
Cardinal Red '88 GT 5 speed, 99 Contour Electric Dual Fans, Battery relocated, 130 amp alternator, 76mm Mass Air conversion, 75mm BBK throttle body, 5.0L .030 over, BBK SSI, AFR 185cc heads, Comp Cams XE276HR, BBK ceramic longtubes, Spec Stage II clutch, MM front/rear coil overs, MM front LCA's, MM manual brake conversion, Flaming River Manual Rack, 94-04 Cobra front brakes, 99 Cobra IRS, Torsen T2R differential, 3:55 gears