Why is my top speed 70 mph?

I could go well over 100 with my 318 / 904 and 4.10 gears. The engine sounding like it's going 120 and only doing 70 sounds to me like transmission slippage. Check the fluid, look for clean, red fluid with no burned smell. Pull the transmission pan if you have the inclination. Look for excessive metal on the magnet and a dark film / sludge on the bottom of the pan from worn material. If any of those are apparent, it is not good, but change the filter and fluid anyway and drive it again.