Welcome to the 2.6 perennial mystery club. Are we talking hesitation on a cold start or are we talking a warm engine? I'd start by verifying all the vac hoses are in-place and tight, move on to the EGR circuit to determine that it's not fouled and conclude with plugs, wires, distributor. This is presuming you have timed the engine and adjusted the valves as part of putting it all back together. It's a common problem which frequently defies instant solutions. It's a process of elimination. If the mechanical side doesn't yield a solution the electronics are next.