How to replace InterMountain trucks?

Leave them off.  They are used to adjust the ride height of the car above the trucks.  The car will be high enough already.  Actually it will ride a little too high due to the fact that the 3-rail coupler arm acts like a spacer.  I usually cut the bolster on the car thinner to lower the car a little.  By the way, Intermountain supplies Weaver plastic trucks and couplers.  You can directly substitute Weaver diecast trucks and couplers if you want.

Thanks for the quick reply!  I'll leave them off.  Will pick up the metal Weaver trucks and couplers as the plastic couplers seem to hang up a but.

Don't get me started on Weaver plastic couplers!

But, from an experienced truck-swapper, and for future reference: their die cast are not the best, but are a vast improvement over the plastic (the plastic material is not

the problem, as the plastic truck itself is just fine; the "spring" and incorrect knuckle movement arc are).

BTW, the die cast couplers can be purchased separately from the trucks,

and fit the plastic trucks, of course, making your conversions cheaper. Then paint

the whole shebang with flat spray camo brown from a hardware store. Looks like a dirty 

freight car truck.