Will I have any idle problems with upgrading to 750cc and a new tune to 91 pump gas? Right now I have 560cc with e40 with a 255 walbro running 26lbs on a 68 HTA turbo.

What are you tuning with?

Dsmlink V3

Should be no problem at all.

Well with that answer I'm assuming no problem with 850 as well. So I wouldn't run out of pump as long as I have a proper tune then? Just want to know if I ever want to go back to e85 or e40 which I have now

You might be pushing the limits of the pump on a 255lph at 26psi running e85. Make sure the voltage to the pump is as strong as possible and make sure the fuel supply line is without restriction as much as possible. Otherwise upgrade the pump. When you turn the wick up, watch the AFR under higher boost levels to make sure the fuel keeps up and you don't go lean.