Sorry, no clue about taller gear, but a ST 90 and a 15x4 (or 16x4) APC should be a beast. Plan on a bunch of tail weight. My Ultimate FF flew with a 16 oz (with muffler) Rossi and needed 1 ounce of lead to fly nice. Not a 3-D set up at all. The RX is intended to be in the LE of the right wing, behind the engine, but put it where ever you can. The batt goes in the left wing LE (or elsewhere). A Saito 91 is probably a better choice, maybe even a Saito 72, but I understand you already have the ST. The OS 61 FX is a bit lighter than a ST 90 too. Either way look for a Jett Engineering Muffler to enhance power to overcome your altitude problem. While building, entertain the thought of increasing spacing of ribs to make the wing bigger, to help with altitudte too. Altitude affects the way wing loading works too. The Ultimate FF flies great at 4.0 to 4.5 pounds and 6 to 8 pounds of thrust at sea level. Work numbers on thrust and wing loading to your altitude. Maybe try to cut an inch or so off of your nose too. The ST 90 is much more motor (heavier) than intended. Good luck!