So I'm looking at getting a shift kit so my transmission will stand up to my new 350 powerhouse and was wondering what wisdom TGO had in the matter. I don't expect it to hold up for long, but long enough for a T56. The new motor is about 450 Horse, and a little over 450 torque.

 

Shift kits don't increase torque capacity. In fact, your trans will live linger without a kit making the 1-2 shift more brutal. That tends to cause second to fail, and takes OD out with it. Just do a new filter and fluid. If you really want to try more line pressure, TCI sells the pressure regulator spring separately, but be aware that TransGo's shift kit doesn't even increase the line pressure.
Best to just do the filter, baby the 700, then sell it as a still-fully-functional unit when you start on the stick.