T5 should bolt right up. Is it a 1980 Dana 300? If so I think it's considered a long 300 (1980 only, I have one too). I think 1981 and up are a little different, but overall are interchangeable. It will only matter when ordering parts.
I've had great luck with my 300. I rebuilt it with a JB LowMax 4:1 gear reduction kit and that thing crawls over everything I need it to. Overall the Dana 300 is pretty easy to rebuild. I got the Moses Ludel book on jeep cj rebuilding. It was $30 but well worth it. I have the T-176 and I think it's the same bolt pattern as the T5 (assuming you have a jeep T5). So, it should bolt right up.