Is it possible to make the automatic transmission attached to the 300D work with the kick down cable and vacuum line disconnected? I would be ok with manual shifting all the time. I know it could be done with some GM trannys but not certain of the procedure.

Bowden cable can perhaps be disconnected, but you really need the vacuum for the modulator, even if you plan to do all the shifting yourself.

The vacuum line affects the modulator pressure inside the transmission, which is critical for operation, and not just to determine shift points.

Too much modulator pressure while shifting and you wear out all the parts in the drivetrain with the stress of the very hard shifts, in addition to more wear of the transmission hard parts. Too little modulator pressure while running gets you slippage all the time, and severe transmission band/clutch wear can occur very quickly.

I hate to reply to myself, but I just had what may be a deliciousely evil idea.

How about a clutch pedal, but hooked to the vacuum valve, instead of having it on the throttle?