Does the intake manifold need to come off in order to change the thermostat?
No the manifold does not need to come off to replace, but the cowl does.
part # for gasket to block #5245643/requires 2.
part # for t-stat gasket #4763839.
Chrysler recommends dexron IIe or dexron III for transmission fluid change but a tag on the side of the transmission should have the actual # for Castrol Syntorq which is a synthetic transmission fluid. Chrysler's parts catalogs do not list the part # for this Castrol, but the # on the trans is available through Mopar parts and has an Amsoil equivalent.
If and when you replace the t-stat, make sure to replace the hose clamps on the housing and you may thinka about replacing the hoses at the same time. Sean Roe makes a very nice set of silicone hoses that fit beautifully and come in a couple different colors and have a bleeder screw built into the upper radiator hose that makes bleeding air easier/good luck.