GM transmission bolt pattern may be the same but the heavier duty setups often used larger diameter flywheels than the same engine in a small truck, bigger clutches. And where you can get into trouble trying to swap is longer (often larger too) input shafts in the transmission.
You can machine a spacer or if available swap the smaller input shaft to get the transmission from a big truck to fit a small, or rather shallow, bellhousing.
Other way around putting a small truck transmission with common input shaft on a big truck bellhousing can result in the input shaft being too short to reach the pilot bearing, might not be anything to do about that... swap input shafts if you can.
Advance Adapters may have the spacer and I'm sure they have the input retainer adapter... what is it, 4-11/16" to 5-1/8" ?
