1/4"npt seems small to me, i guess i was lucky my heater went in as you said up top, to the intake man.
Try daves article here: http://www.wakeworld.com/articles/2002/heater.asp#Engine
The first fitting will go somewhere on the intake manifold toward the front of the engine or on the thermostat itself. Hopefully, you'll have a cooling system plug that can be removed to accommodate the 5/8" barbed fitting provided. If not, you may have to tee into the temperature-sending unit. Be sure to use Teflon tape on all your threads in order to prevent leaks.