1st gear Hesitation

I am guessing timming but what the hell do I know LOL
73 Cuda 383 4 speed.
The car has been and still is running great, but I have now noticed that under hard acceleration in 1st it sputters and losses power at a high rpm, 3500 plus. If I take off normally it is fine. This only happens in First gear.
Thanks
AL
Author: ram250098