If the water heater has a side T&P location, there may be two 3/4" plugs in the top of the unit. One may be the anode and the other could (local ordinances/rules/laws/manufacturer permitting) be used as an alternate location for the T&P valve. If this is the case, leave the T&P on the side and just use one of the two top plugs for the loop.
If the only way to tie in the water heater into the circulation system is to remove the T&P and not have it directly tied into the water heater, a water heater should be purchased that has provision for the T&P to be threaded directly in to the tank.
One of the big water heater manufacturers really frowns on any alterations to the T&P valve. It will void the warranty. Many/most water heater manufacturers have been installing T&P valves at the factory, and shipping them that way for a few years.