transmission problem

[Weezy]

I'd put my car in reverse and it won't stay. It'll only stay when i start to let go of clutch and gas it. Sometimes I'd be driving and as I'm deaccelerating, it's come out of gear. I need some help here. If i need a new transmission, i have a spare ka24de one. I heard that i can use it as long as i swap bell housings, would it be hard to swap bell housings? thanks in advance.