I don't think the CLT restriction is issued anymore; you get a standard MEL even if you're getting the rating in a Skymaster.
I agree with the suggestion made above, i.e., get the MEL in a standard multi-engine airplane first for less aggravation. I believe they have Duchesses/instructors at Livermore, San Carlos, and Reid-Hillview.
There used to be a 337 parked at Livermore, one at Concord, and one at Reid-Hillview. You might want to see if they are still there and ask if they'd recommend a CFI specifically for the 337 if you decide to train only in the 337.
Anton Sabev at
www.pilotratings.com might be able to help you, too. I have used Anton for many years and he is a great instructor on almost any airframe, working out of the greater Bay Area airports and flight clubs/schools. Phone was 650-776-4435 (don't know if that's current.)