RE: Dynaflite 81" Spitfire
I built (and rebuilt due to incidents) mine according to the plans, no retracts, covered with Monokote and used a Saito 150, which is too much power at full throttle. A 120 would fly mine just fine.
I agree about beefing up the stab, I got flutter until I rebuilt it and added a 1/8" plywood full width spar. No problems since.