Transmission issues

[hawkfn18]

First the backstory.  I have a 1973 Challenger with a 340 and 727 trans.  Back in 2000 I lost my job, house and everything but the Challenger.  I had given the car to a family member free of charge because he always loved it and wanted to restore it.  I figured at least it would stay in the family.  Fast forward 15 years, back on my feet, married, house, financially stable etc.  Caught the Mopar bug again and the family member that I had given the Challenger to had pretty much used the car as a storage shelf but had the transmission rebuilt.  I asked if he would sell me the car back, the cost being what he put into it.  It hadn't been on the road in 15 years, so after replacing all the lines on the car, suspension work and other misc stuff I got the car back on the road.  Fought with the kickdown linkage and finally got that adjusted correctly last year.  No idea what was done with the rebuild, shifts were firm and would bark the tires every now and then. 

All was good until yesterday.  It seems now that only reverse and low gear works, will not shift gears when driving in drive or while trying to manually shift through the gears.  The rpms also will crash when coming to a stop, the car almost dies out.  Fluid level is good and bright red.  I haven't dropped the pan yet to see if there is anything bad in it.  Is this a converter issue?  Kickdown band adjustment needed or is the trans toast?  Transmission stuff has always been "dark arts" to me.  Any input would be greatly appreciated guys.  Thanks in advance!