I've got a rebuilt Marlin trans in my truck and a URD short shift kit and ive got a few negative things to say about marlin. First off, marlins customer support is non-existent. they never answer calls or emails, it took me a good 50-60 phone calls to finally reach them. Secondly, when my transmission arrived, BOTH the fill plug and the drain plug were Cross-threaded on and stripped. Big headache...
The transmission itself though, has been great. Shifts like butter and is whisper quiet.
I say find a local shop around you and have it rebuilt.