Bought a 1985 305 TPI auto(has all the goodies!) Trans Am a few years which surprising has the original numbers matching motor and trans but after alot of hard work and finally took it for a test drive only to find out the transmission would not go past first gear,went thru all the troubleshooting info only to no avail.Recently purchased another 700R4,was told it was out of an 1988 Suburban,come to find the stamped numbers indicated it was a 1990 and no vin numbers available to actually know what it was out of.Did some research and found out it was the better and stronger 700R4.One thing I did overlook was that the speedo connector was vss not mechanical(I have to switch the output shaft housings).want to convert to electronic but after some net research I think it would best to swap the gears out(I found that the output shaft has the hole already drilled into it for the clip for the plastic gear).My question is-where can I find an mechanical to electronic adapter that will work with factory vss?

Do what now? You lost me at "mechanical to electronic adapter". Just swap the tailshaft and speedo drive gears off your stock trans.

Unless the new/used trans has been rebuilt recently, plan accordingly for it to not function correctly. Maybe your luck will be better than mine, but at one point I had half a dozen junk 700r4s under my bench that supposedly worked fine according to the sellers.

An alternative answer for the vss
http://tpiparts.net/inc/sdetail/7587
Assuming your using the 1990 1227730 ecm.

out of a suburban, tail housing has to be swapped anyway.