taking your sled and running it in 700 improved class with stock pipes is like taking a pocket knife to a gun fight!
No matter how much work you put into the engine your gonna be bound by the limit of the stock pipes, and thats right about 165hp!
MPH is indictive of your hp so the lower your hp, the lower your mph will be.
I would just stick to fun runs and leave the class racing alone till you get the rest of the parts to run that class, the big bore pipes, bored or oversize carbs,modded reed cages,etc.