I have a mower with a 5.5 hp briggs engine i pulled the carb and gas tank to replace the tank (rust in tank hole formed). Cleaned out the carb and put in a new gasket and diaphragm. But now it leaks gas where the carb mounts to the engine (tube going to the intake) i can see it leaking there. I put the o ring and the plastic piece that keeps it in place back? Engine runs better but sputters here and there leaks gas when it does (assume a lean condition). Any ideas on the leak? Bought a new seal as well.

You mean its leaking while running or setting. I had a push mower that started and ran just fine, but setting with gas in the tank, it would leak out the whole tank in a few hours. I just put a manual shut off valve in the hose between the carb and tank.

No only while running and this was a briggs and stratton push mower where the carb sits ontop of the tank. I solved the issues by swapping carbs from another mower even though i swapped seals first and that didnt help? I dont know why it worked but it did.