as far as the nacelles go .use 1/8 or 3/16 birch ply for a firewall and mount an engine mount to it then the engine to the mount.with the stringers and sheeting it should be fine with the .25.but an 87" span plane using twin .25's may be a bit underpowered.my duelist has a 60 = span and uses 2 hp 40's weighs 10+ pounds with retracts.by stafford b-24 has 90' span weighs 13 + pounds use 4 fox .25 with 9 servos.i would think something in the .45-.50 range would be about right unless your plane weighs under about 8 pounds.