I put a JR-341 in the tail of my 3.7 meter DG-800 a few years ago, and its far from the strongest or fastest mini servo available. Initially I was worried about room, but after I removed the tail post and got in there, I found there was tons of room for the servo and the shortest arm, and I ended up with plenty of throw as well.

Its really basic- make a short pushrod, zero your radio trims and subtrims, clamp the elevator in its neutral position, and put a 1/2" dab of Goop on the four corners of the servo. Put it in place, and let the Goop cure. Next day, turn your radio on and check that everything came out zeroed, and youre ready to go. If the servo needs to come out, a pair of long needle nose pliers will allow you to twist the servo free and try again. If everything looks good, glue your rudder post back in, and reinstall your rudder. I had a very minor change in CG as well.