Originally posted by R Pope
You must have knocked something out of kilter when installing the wrap. Heat retention in the headers won't affect shift points. Check linkage, vacuum lines, etc.
I'll second that. I'll bet you accidentally knocked off your vacuum line that goes to your tranny vacuum modulator........ assuming a GM tranny is similar to my Ford C6, in that they use a vacuum modulator to shift If everything is hooked up, look for cracks in the vacuum line.