You can top off with Dexron. For a fluid change, use ATF4.
I always use Mopar trans fluid in my street cars. The one time I used dexron, I melted the clutch plate in the torque converter.
In my race 727 (non-lockup) I use Type F designed for fords.
Your gonna get alot of opinions from people but why not use what Ma Mopar specs out for the intended application?
Not just that, but Dexron was really only recommended for top off anyway. Before ATF+ became standard fair, 7176 was name of the game. Then that was replaced with the ATF's consecutively.
Truthfully, to me, it sounds like you're not getting enough pump pressure. Reverse is usually the first thing to go, even with low fluid.
Try giving it a flush and a filter. It's gonna take a couple of cases of trans fluid, but you can do it el-cheapo and give it a try before you hafta spend long green on a build.
Disconnect the cooler hoses and run 'em into a bucket then crank the car. I've seen 'em pump out in just a few seconds. This empties the convertor and the lines, too. Hook your lines back up and fill. Run to operating temp and slowly -with foot on brake - shift through the gears. Top off like you normally would. Then, pull the hoses, and repeat to flush that fluid out. Basically, you're doing this to pick up and debris a flush it out, removes traces of any "off-brand" fluid which may have used over the years, and it saves you from taking a bath when you drop the pan. You can buy a can of "cooler flush" at your favorite parts store. Blow that through your cooler before you put your lines back on for the last time.
Change your filter and adjust your bands. Important! Bands have to be adjusted with every trans service. Personally, I never use a pan gasket when putting the pan back on. Permatex makes a grey RTV gasket maker that dries under torque. Use it instead of the black and throw the neoprene gasket over your shoulder and forget you ever saw it. Hell, Chrysler doesn't even use a gasket on the assembly line.
Refill with ATF4, let it get to operating temp and, again, slowly shift through the gears. Top off per the dipstick marks and test drive.
Will it work? Maybe, maybe not. After years of abuse (no one ever thinks to service the transmission every 30,000 miles- but they'll change engine oil every 3000 miles) really, for less than a $100 in material, it's worth a shot to give it a try.