If you ditch the pants you can probably go up in wheel size to accomidate larger props. If all else fails the gear should be very easy to make with a longer peice of alum. As for three blade props I cannot help you there.

I sympathise with the problem with the altitude in your area. I think the trouble with getting through should be considered on your end. You are asking for advice to go down a path that others with this plane are cautioning you against. The 20% deficit would be better made up with a lighter motor and even with the 61 fx you would be advised to shorten the nose about 1/2". The weight you listed earlier on the S90 does not include the heavy monster muffler that comes with the supertiger. They do not like to advertise the weight of it.

We all like power, I currently run a Webra speed 120 on a 60 size extra. If you believe hp #'s the 61fx produces 1 ful hp more than the os 52 on my ultimate (over twice the hp).

Some planes can handle the weight and power better than others before flight performance and overall satisfaction with the model diminishes. Perhaps to avoid adding tail weight you can come up with something more creative. How about a 2 foot tail boom to mount the battery pack on?