Mark no disrespect intended, but a 6" pitch prop of any diameter or variance in blade shape is simply too much to be used sensibly with that airframe even with a more powerful ballraced .46. The overiding factor in this case is the limitation imposed by the airframe.

Airframe drag of the LT-40, always relatively high, worsens expotentially as speed increases. ie: V². Thus both by design intention and the limitation imposed by aerodynamic laws upon its optimum operating envelope is to remain in the low speed flight regime to be efficient. Because of that due its its size, aerofoil and consequential parasite or form and induced drag inter-relationship, it requires a lowish pitch prop for propellor and flight envelope efficiency.

Engine, airframe and propellor acceleration are inter-related. Thus small diameter or high pitch props, the combination of the two being the worst, are inefficient in the LT-40 scenario proposed. Too small a diameter doesn't produce enough accelerative thrust because of slippage losses failing to accelerate that small mass of fair to sufficiently high a speed vs airframe weight & drag. Similarly the consequence of using too large a diameter where the prop again becomes innefficient through its inability to accelerate the now larger mass of air against the drag of the airframe without loss again resulting in slippage and prop inefficiency. Increasing pitch in conjunction with the lower RPM imposition of the increased blade diameter similarly exacerbates this effect as it did with the too small a diameter. Exacerbating it further with a highly timed 2 stroke like the TT46, you're loading it below its optimum torque and power curves resulting in totally porked acceleration and power delivery.

Now putting that into perspective here, a 12x6 is a big ask of a .46 at any time, and for this combo, if you'll pardon me saying so, unwise. Or translating that into non-techno speak, "pulling power" is exactly what it won't deliver. A 12x4 narrow blade might just work, but the optimum efficiency for the combo is undoubtedly sized between an 11x5 and 11.5 x 5. Propellor brand doesn't matter, though APC and Bolly Sportsman props are both efficient though broader blade designs. Now the TT46 is a good engine, but with only about the equivalent power of the long since superceded OS 46SF. An OS 46FX, AX or Enya CX45 it ain't, and even they'd be bogged down by the sizing suggestions you've recommended. With a 12x6 will it fly? Yes. Will it fly well? No.