The 2004R would be waaaayyyy to light for a 3/4 ton. Even a 700R4 is a tad on the light side, GM only used it on the light duty 3/4 tons, which aren't much more than a HD 1/2 ton.
If you really want an automatic that's properly mathced to a 3/4 ton truck, look into the 4L80, which is standard equipment in all GM 3/4 and 1 ton trucks '07 and newer (and was optional from '94-'07). There's conversion kits out to deal with the electronics, the only real isse would be adapting it to fit the Poncho engine. May need to have an adapter palte fabricated, not only to mathc the bolt pattern, but also to provide a rear engine mounting.
