Guess it really depends on what class you are going to run. Sounds like SP.
My first question is why. Run what everybody else is running. There is no need to reinvent the wheel. I'm no engine "expert" but I have never seen or heard a 25 with a 28 crank. I have heard alot of talk about them here, but never seen one at the track. You can build a killer motor out of a 28. Be it a 28, 31, or bigger. I have a 28 in mine and I took 6th and 3rd at Mendota, I was ahead of many 31's and I think there were some bigger engines in the field too.
Give George or Don a call, they both can hook you up with something that will smoke a experimental motor and run a long time. It all really depends on how much money you are willing to spend as to how fast you are going to go.
And seeing that you are 14 I wouldn't recommend a experimental motor. Sometimes they go BOOM!