Here's the deal with 5.0s from 1985-1995...with manuals they are geared to run the quarter in third gear, which depending upon the year (1985-1989 have 1.29 third gears and 1990-1995 have 1.34) equals out to 3.97 and 4.12 respectively with 3.08:1 gears.
A stock convertible auto puts you a distinct dis-advantage, one being added weight, the other being that Ford didn't bother to gear the auto for drag racing at all.
Assuming you want to drag with it...even 4.10s won't give you optimum gearing for the quarter...you'll need 4.30s, due to extra weight and the slush factor in the auto. On the positive side, you do have overdrive, which makes even the 4.30s comfortable on the freeway. With a stock engine in a stock car auto/convertible (with stock height tires) you can expect a best of about 15.0 @ 90 with optimum gearing and great traction and driving.
As far as your next step...as long as your engine hasn't been abused (poor maintenance schedule and/or over revving and/or detonation) your bottom end should be pretty freaking stout. I know a guy who was running his 94 5.0 with 200,000 miles and nitrous at the track every weekend without any trouble. I would imagine your engine with new valve springs and timing chain (assuming it was in good shape) would be plenty happy with a small amount of boost on it. Which, by the way, would also change your gearing selection choice, bringing the required gearing down a notch. The approximate change (assuming that the supercharged engine was otherwise stock and the boost was down at 6 psi, allowing the engine to make around 300 HP) would be down to approximately 4.10 or even 3.90 (depending upon whether you wanted to trap at around 5200 or 5600 RPM).
