Are you planning to keep the spicer 23 transfer case?
I have not done a swap in a Napco, but I have done a swap in a 1959 gmc behind a 1959 pontiac 389, and another in a 1940 gmc with a 302 6 cylinder.
Getting back to your swap... 6 cylinder and v8 may make a difference in how you tackle the swap. You might be able to find a v8 bellhousing that is correct for the NV4500. The trick is to find one that bolts to the crossmember like original.
In both of my swaps, I used a 1994-1997 dodge 2wd nv4500. I used this version for 2 main reasons.
1. The pilot shaft is one inch longer than the gm version. This allows you to simply use an adapter plate that is 1 inch thick between the transmission and bellhousing. Most of the stock clutch linkage and components can be reused, including the clutch disc, pressure plate, and throwout bearing with some machining. A custom pilot bearing will be necessary.[NOTE 1: Advance Adapter sells an adapter plate, but further machining is required for it to bolt up to a stock bedllhousing. Either the bellhousing needs to be bored out to fit the plate, or the indexing lip on the plate needs to be turned down to fit the bellhousing.] [Note 2: There is a crossover throwour bearing that is available. If you would like to run a stock throwout bearing, the bearing retainer on the transmission will have to be machined down to fit the throwout bearing. This can be done on a metal cutting lathe.
2. The 94-97 dodge 2wd version can be made to work with a mechanical speedometer. In your case, the speedometer hooks up to the transfercase, so the speedo hookup isn't critical.
The early chevy nv4500 transmissions have the lowest 1st gear of 6.34:1. They also have a bellhousing bolt pattern that is unique to the early chevy NV4500, The Chevy 2wd NV4500 is shorter than the dodge 2wd NV4500. It may be possible to shorten the driveshaft from the transmission to the transfercase using the 2wd chevy transmission.
If you use any dodge 2wd NV4500, you will likely have to relocate the transfercase back, which means you will need to have work done on all 3 driveshafts.
The 4wd NV4500s are shorter than the 2wd units. They are intended to be married to the transfer case. I do not know if it is possible to bolt a yoke to the 4wd transmission and run a shaft from it to the transfer case. There are companies out there that sell adapters to bolt the NV4500 to numerous transfer cases. You would want a transfer case with a passenger drop if I remember correctly. I believe the 4wd NV4500 trucks used a Velocity sensor in the rear axle. I do not know if there is a speedo pinion gear in the newer transfer cases.
I found these sites to have some usefull information and/or parts.
http://www.high-impact.net/transmission_and_gear/nv4500.htmhttp://www.novak-adapt.com/knowledge/nv4500.htmwww.advanceadapters.com/products/nv4500-transmissions
