One option, used by some early -6 builders, is to buy a shorter push-pull cable and mount the Ray Allen trim servo on the flat deck under the fiberglass tail fairing.
This way there is no modification to the metal work of the horizontal tail.
This is what I did when I converted from manual trim to electric trim. It works great. The push-pull cable needs to be about 28 to 30 inches in length for this to work optimally. All cable parts can be purchased through Aircraft Spruce.
I highly recommend using Pat Hatches “Ultimate Trim Relay Board (UTRB)” and mounting it under the copilot’s seat. The use of a trim relay board prevents both pilot positions from inputting trim control inputs at the same time thus either cancelling each other or worse burning up the trim control motor(s).
At the time I converted my aileron and elevator trim from manual to electric I also installed five button Tosten military style grips. (Tosten is another great vendor who will help you think through the grip/button issues.) The UTRB board really helped in the wiring up of the additional switches. My grips now have buttons for PTT, ident, flaps/com swap - depending whether it’s the pilot or copilot stick, freq swap and autopilot engage/disengage. This makes flying real easy and I can pretend its a military jet trainer ie T37 or T38. (I loved flying the T38 when I was a youngster.)
Its sounds like a major wiring job but it actually wasn’t too bad with Pat Hatch’s assistance. He guided me through the process via emails and a few phone calls. He lives in Florida and I live in Oklahoma and my electrical knowledge is minimal. (Accountant by education - good with money/dumb with sparks!) However, Pat still successfully guided me through this process pretty easily. The actual work took two plus long weeks of almost daily effort but I absolutely love the results. By the way, I don’t use a speed control switch on my elevator trim. Didn’t need it.
PS: Document via schematics, pictures and written notes this change. You may need to modify or repair something later on. I also highly recommend getting a small heat printer at Office Depot or some place else plus shrink tubing and label all wiring you install or hook up. Steinair has some great UTube videos on how to do aircraft wiring and will also sell you the tools, wire and connectors you’ll need.
Don’t be intimidated by the project. It’s a very doable and well worth the effort.
Update: share before and after photos..