? on C6 trans

I am working on a 1969 F100 with a 390 and c6 trans for a lady this truck had been sitting for the last few years got it running great now. I noticed it shift really hard into reverse but into drive it shifts right asked the Lady if it did that before it was parked she said yes that the reason she parked it. When I drive it shifts right with no slipping. I did find the trans mount was loose tighten it back up and check engine mount they are good checked u joints and center bearing all is good. The truck Idles at 600 rpm now in park so it not idled to high it was idling around 800 in park after I idled it down it helped a tad but any lower it will die when put in gear. I have noticed it wants to move forward when I crank it up unless I put it in neutral and it doesn't try to move. Would this be a problem with the valve body or band problem.

The harsh shift into reverse could be in the accummulation port in the VB. Sloppy or worn valve in that position.

I was talking with the lady she said not long after her uncle put the radiator in it the trans cooler in it went to leaking and filled the trans full of antifreeze. She said she carried it to a transmission shop when she lived in Atlanta Ga. they did a trans flush and put a new filter and fluid in it. She said it started acting up right after she moved down here and that's the reason she parked it. I might pull the valve body and check it out or have her carry it to a transmission shop and get it rebuilt.

I put some Locus transmission additive in it now it's not slamming into reverse now. She is happy with it told her if it starts back to carry it to a transmission shop as I don't work on transmissions.