How high does the engine Rev? If something (sol vac or other related engine management system) is holding the throttle open, you could disable it. But if I remember right you arn't running a carter feedback carb.

That's the problem with automatic transmissions and low gear. The torque converter that lets the engine rev up before it stalls giving you that little boost off the line, also lets the drive train rev up before there is significant torque on the engine. On modern cars with a lock up torque converter you can hot wire the TCC (torque converter clutch) and get about the same effect as a manual transmission. Unfortunately, your 3 speed auto doesn't have a TCC.

You can swap out the 3 speed auto for a 5 speed manual... How much is down hill engine braking worth to you?