Jack-

Every time it has siezed up it has been in the midrange. Piston clearances are within spec. I'm using the stock carb with no other adjustments to it except being bored by RCR, a bigger main jet, and a lowered needle clip by 1 notch. Do I need to make other adjustments? What is a good place / method to look for air leaks?

 

"Every time it has siezed up it has been in the midrange..... I'm using the stock carb with no other adjustments to it except being bored by RCR,"

+ resulting in more air flow

"....and a lowered needle clip by 1 notch. Do I need to make other adjustments?"

+ Yes, since the engine is melting down at Mid-RangeRPM levels you are running too lean in this area which is controlled by the needle adjustment. It appears that in your case, 1 notch down is too much.
What is a good place / method to look for air leaks?

+ pressurizing the crankcase is the best way to check for these leaks looking for soap bubbles in the seal and gasket areas.