You don't want to go 70+ on NiMh batteries, thats for sure. Invest in LiPo before you decide to do speed runs.
Also, how much money are you willing to spend to go 70mph? A MMM2200 combo can push it to 70mph, but not for long. Whereas you can pump a Neu 1527 in there and 70mph for longer run times.
But I will say 70mph gets old fast just going back and forth... LOL