[brian]

I just replaced mine last week. I got tired about worrying about having air pressure to flush the toilet.
I bought a flange for the floor that the toilet bolts to that required a 4" hole, I was able to drill the hole Offset from the original smaller hole to move it over about a 1".
That gave me enough room to mount the flange that bolts to the top of the tank, the flange on the tank   
Requires a 3" hole to be drilled in the tank, I was able to drill it straight down from the 4 " hole.
I drilled a 4" hole in a scrap piece of 3/4" plywood to use as a guide for the hole saw and screwed it to the floor where I wanted to drill the hole. Hopefully that will give you enough room.
It worked on mine. You call p.m. For my phone # if you want to talk about it.
We went camping at choke canyon lake last weekend and I am loving my new toilet. Paul

Thanks Paul for your insight and experience.

I have a 3 inch hole in the floor already.  I'm leaning to use the plywood guide trick and expand the current 1.5 inche waste hole by turning the hole saw by hand since there is not enough room to accomodate a drill.

Although, I wondered if one were to flush often enough, if one could attach 3 inch piping to the flange which runs vertically for 7 inches or so then tie into the existing 1.5 inch pipe that runs horizontally for 5 or inches so as not to deal with the existing tank hole. I know for a fact that my microphor did not have the recommended 60 psi (possibly 30 or less) and the effluent ran into the tank.  Does the hopper act as a mini macerator?

Yes, I do know the above proposed scenario is not to code.