My 72" Hellcat came out to 19 lbs. With the relatively short nose I did every thing I could to shift weight forward. Has a DLE 35RA. I built platforms inside the cowl to mount a 3,000mha battery for the RX and a 2,500 mha battery for the ignition. Also used a heavy brass Hamilton Hub and a heavy brass nut from Higley. I addition, I cheated and moved the engine and cowl about a half inch further forward. I still had to put about 9oz of lead shot in the lip of the cowl. Flies ok. The 35RA is enough, but not over powered.

I know you don't have room for the batteries in the engine compartment of the Mustang. However, with that long nose it surprises me that it is tail heavy. I have a 60 size Top Flite Mustang with a DLE 20. I had to mount one battery all the way back, just in front of the tail wheel to get the correct CG. Start out with the CG back 25% of MAC. That should be controllable and then you can adjust from there.