Another approach if you didn't mind having really firm shifts would be to shift the trans manually, it would take a custom built shifter with 2 HD mirco switches that would operate the shift solenoids according to the GM manual shift sequence.
With a some thought one could build a circut to monitor a TPS sesnor and then PWM the line pressure solenoid. 0% (off) = full line pressure ~75% = idle line pressure. A vac. switch could be used to lock up the converter when in 4th.
How handy are you with electronics?
