Transmission reverse not working properly

So I have the car listed above 305 CSB fully rebuilt runs and idles great. Transmission will shift gears fine while driving. When put into reverse it will rev up and barely move. Reverses alright on flat ground but with any load it won’t reverse. It will rev up as if it’s in neutral. It’s leaking trans fluid not sure where from. Even when the car is filled up with fluid I still have the same problem in reverse. Also it makes a weird scraping/whining noise only in reverse. Replaced the trans filter and a new gasket on the trans pan. Any ideas about what’s going on, I searched the whole internet and can’t find any clues. I’m very reluctant to bring it into a shop because i’ve pretty much done all the work you can think of to this car. Thanks for the help!