Why is my top speed 70 mph?

Over revving in 1st/2nd gear as mentioned by the OP....incorrect throttle pressure linkage (kickdown) adjustment, and may have already burnt up the trans from how many miles of driving it this way!! That would be my guess, trans slipping/toast!! May not even be hooked up!!

How about a pic of the carb area? That may shed some light on this! Geof