727 torque convertor swap

If you don't know the history of the trans, I would remove the pan first for inspection & to drain fluid before you put a new T/C in, if looks good install a shift kit and replace all external seals when it's out, flush the trans cooler & lines, if you find debris in the pan it's time for a rebuild with your new T/C.
One more thing use some blue lock-tight on the T/C bolts and torque them to specification. I learned from that mistake back when I was 17.