Mel, as I see it, you are in kind of a difficult situation here. You don't plan to do the work yourself, and I can fully understand the reluctance of your shop to do a job that they don't feel good about warranting. At first I was going to suggest that you look for a shop which is more focused on rod-type projects, but then I re-read your earlier post and saw that you have one of those now.
For that reason, I am confused about some of the other statements in your post:
"The shop that I am using that does many transmissions is unable to get an adapter for the Dodge NV4500. They spoke with the adapter folks and it does not fit and would require a lot of tweaking."
Does this mean that Advance can not supply the adapter which I suggested (P/N 712550), or does it mean that Advance has the part, but does not believe it will work for your application? If the latter, why do they say it won't work?
In my earlier post, maybe I wasn't clear enough about the swap not being a bolt-in. Perhaps I should have posted the link to the Advance instruction sheet at that time, but I didn't, so here it is now:
http://www.advanceadapters.com/downloads/712550.pdf
I have done this swap (on a Chevy, not a GMC), and the adapter absolutely does fit as described, but some machine shop work is required. First, on any bellhousing 1967 or older, the center hole has to be opened up to accept the 5.125" register diameter on the adapter. Second, I didn't want to use the suggested Ford release bearing because I was concerned about a potential interface issue between the bearing and the stock fork. Instead, I machined the Dodge release bearing support to fit a stock GM bearing (about 1.375"). Lastly, I had to modify the driveshaft, but I think you expected that anyway.
Naturally, all of this is dependent on using the correct transmission. It has to be for a Dodge, not GM, and it has to be for a gas engine, not diesel. All of this is very easy to verify if you can measure the input shaft. It has to stick out about 7-1/2" from the transmission face, and it must have a 1-1/8" spline diameter (same as a stock GM clutch).
I see that Advance now sells the correct Dodge input shaft:
http://www.advanceadapters.com/produ...s-input-shaft/
However, I don't know is this is to allow the use of a GM transmission with their adapter, or if it is sold only for replacement purposes in a Dodge transmission. You will need to ask Advance about this. If it does work in a GM transmission, this will make the swap somewhat easier.
As an alternative to the NV4500, you are correct that the MY6 (A-833) is an easier install. It is certainly less expensive than an NV4500 and will save you the cost of the adapter. You also save quite a bit of weight. However, it still isn't a bolt-in. You will need to machine the bellhousing the same as for the NV4500. Also, the case was made with two different bolt patterns and only one of them will work. Lastly, the correct shifter can be a real PITA to find if one doesn't come with the transmission.
The conventional 4-speed (like an SM420) with a taller rear axle would be a distant 3rd choice. First, the good news: The SM420 is is almost a bolt-in swap. It will bolt to your bellhousing with no modifications at all because it was a factory option. This also means that you could very well find the correct driveshaft for it at your friendly local salvage yard. All you may need to do is cut the hole in the floor.
Now the downside: You may not be able to get tall enough gears for your rear end. At this point we don't even know what kind of rear you have, so you will need to identify it first before you can go looking for gears. If no luck on the gears you will have to swap in a rear from a later year truck. It has to be approximately the same width as the rear you have now, because narrowing a rear can get expensive. Fortunately, Ford and Dodge also used the same 8 bolt wheel pattern, so you should have a fairly good selection to choose from besides GM. Keep in mind that just about any rear end swap will require relocating the spring pads and also driveshaft modification. You will also have to cobble the parking brake hookup unless you get an SM420 that has an integral parking brake. Lastly, the 3-4 gap on the SM420 is quite a bit wider than on the MY6 (or 4-5 on the NV4500). This means you may not have an entirely suitable gear for climbing hills at highway speeds when fully loaded.
Ray