Stiff accelerator pedal

I just installed a 1967 date-coded, low-mileage 283 and a rebuilt Powerglide in my '67 El Camino, but it is very unpleasant to drive due to the stiff accelerator pedal. There is so much resistance that a short drive is a major workout. I have no idea what could be causing this, but I hope it is an easy fix. I also had a lot of throttle and carburetor work done, and I am NOT a mechanic by any means. Thanks in advance.