My A-body did that. I believe the 727 in it was a '72 vintage. It shifted too fast through all gears. It did not shift crisply into second gear and also had what seemed to be a small amount of drag when shifting at full throttle from first to second.

I put a shift kit in it and it helped the shifts become more positive but as I recall it did not do a whole lot for extending out the time between shifts.

I have a '74 Trailduster and it up shifts and downshifts far better than earlier types I have had. I have heard that right around that year (74) Chrysler revised their throttle bodies to work better.

The 70 throttle body will probably behave the same way the current one in your GTX is behaving. I would look for one out of a '74 and later piece or find out how to make the old ones work better.