Do you have to use the mamba motors? The novak 3.5r went 79 mph one year at the worlds fastest rc-car challenge on just 2s. Last year the HV4.5 went over 100 mph on 4s and 118 on 5s. These were just off the shelf setups too.
Check out my custom converted 8ight:
Sub 7lb, lowest CG of any 1/8 buggy