Ghostcjc97

Hey guys, this is my first post here, so please bear with me. I've got a 1983 Chevy k10 Short bed Pickup 4wd (350 crate, th350 transmission). The truck originally had a different transmission (I don't remember what it came with, a 4 speed though) that was swapped out waayy before I got it. My problem is that when I put the truck into gear, It takes up to 15 seconds sometimes for it to go into gear. When I am in drive or reverse and come to a complete stop there is a slight shutter. When I take my foot off the brake there is a grinding noise and then the truck seems to go into gear again and will drive. The transmission fluid is good but the tranny leaks bad , I haven't gotten around to replacing seals yet. So I just keep it topped off. I have changed the fluid by the way. I plan to be ripping it out but before I do, I want to have an Idea of what could be wrong. If you guys have any ideas or can refer me to someone for tips, I would greatly appreciate it. Thanks