have to rev it up before l take off. Is that okay?

sounds like you need transmission fluid, or you need the fluid flushed and replaced. It is kind of common in older chevys. The screen gets clogged and the valves get gummy. You should do something soon or you will need to have it rebuilt.

Some of those had flywheel problems, it would crack around the bolt holes, which lets the torque converter creep forward in relation to the tranny fluid pump. After enough time running in this condition, the notches of the converter would be worn off enough to no longer turn the pump at all. If ur fluid level is good, remove the upper transmission cooler line, & crank the engine up, shut it off as soon as it starts- be prepared for one heck of a mess if the pump is turning though! Of course, if no fluid flows, the pump is not running, & all the little chunks of steel worn off the flywheel, torque converter, & fluid pump r laying in the transmission pan.
good luck!