Great info guys thank you all, I think I will go ahead and box back to the firewall area.
I wouldn't stop under the firewall. That is where these frames typically bend upward from normal use. Run the boxing back to where the frame straightens out. I'd go just past the trans crossmember and make a pretty big fishmouth so as not to create a stress riser right there.
I don't know about you guys but these things called an Dodge NV4500 heavy duty don't grow on trees around here. Hard to find used on Craiglist and the junk yards. I may have to shell out some extra cash for reman at a shop.
Yeah, you may want to keep the hinge on your wallet well lubed until you are done with this project. You *can* take a GM 4500 and convert the front of it to a Dodge HD. Depending on the year, the input shaft and input bearing retainer will need to be changed, the retainer pilot will need to be machined to fit the bore of the GM case, the bolt circle on the retainer will need to be modified and the bolt pattern of the transmission will need to be changed. Also, try to get a shifter assembly with the trans. There are several designs of them and they vary in price from a couple to a few hundred, yep, just for the shifter.
Don't forget, in order to run a NV4500 behind a 1st Gen motor, you have to run one of those HIIIIIGH dollar, POS Advance Adapters conversion bellhousing. I say POS because AA does not finish them for ****. Plan on spending a couple hours with a grinder finishing the part. You would think for the money they charge it would be gold plated and polished, but no, it isn't. Buy a good clutch too. I have a South Bend in mine and absolutely love it.
Then you need clutch hydraulics. They are only a few hundred from the dealer though--see things are getting cheaper!! The whole thing comes in a bag and is a drop in install--no bleeding, nothing. Bolt it in and go. This is of course after you make mounting provisions for it.
What transfer case are you going to run?? 241 with a SYE would be a good choice, bolts in, no fuss, no muss. A 205 will work, but you want the round pattern. I have seen some for sale recently and a few for decent prices. I couldn't beg, borrow or steal one when I was doing mine.
On your motor, that cast piece of **** that says Dodge and Cummins on it, throw it away amd make one out of mandrel bent pipe. That piece is a huge restriction and a piece of exhaust pipe will flow better and keep EGTs down a little.