transmission swap?

I bought a 1990 Dakota with 3.9 and auto transmission from my nephew. The transmission is shot and was replaced with a junk yard trans that has no reverse. My question is can I replace with an older 727 trans. I believe the drive shaft will be the wrong length and probably the trans mount will need modified. Any one ever done this? I have had trouble with the A500 transmissions before and dont want to spend the money for a rebuilt one as it would be worth more than the truck. This will not be a daily drive and just used for hauling things now and then so am not concerned about gas milage. Just want to fix for cheap and can get a 727 for next to nothing.

the A-727 can bolt up, the tailshaft of the transmission will be slightly longer than an A-500, I believe and you'll have to either swap tailshafts or get a shorter driveshaft. You'll get check engine lights up the ying-yang b/c the computer is having a hard time trying to engage overdrive & lock-up, but it should work.

Thanks for the reply. Thats about what I thought. May just take out that check engine light to fix that problem, or remove it from the panel and leave under the dash where I can still pull codes with it. I think I will give this a try and see how it goes.