Hi Darrin,
I have a short history lesson. I personally made the original conversion of the Tremec 3550 for the Mustang T-5 replacement into a GM bolt-in. I've sold thousands of these GM Tremecs. The GM mechanical clutch linkages work better than the Mustang cable linkage. Shifting problems are almost always clutch related.
Your Chevelle is a nice conversion. I stock the GM TKO if you need one.