I can vouch for Blumenthal. I know they make in-house or contract locally most of their parts for the NVs. I believe they supply most of the parts available, such as those that Standard or Wade's gets. I've used Bluementhal's over 10 years, and they have always impressed me with their quality, knowledge, and ability to find or build what I need, regardless of the GVW of the vehicle, or it's purpose. I've exchanged Eaton 15 speeds, Getrag 5 speeds, NVs, E4ODs, and many long blocks, and never had but one complaint when a bolt hole was stripped in the output shaft where the yoke bolted on. They DELIVERED me another reman the next day before 12 noon(2. 5 hours away), and the guy helped my pull the first one out of the Pete I was working on before loading it up and leaving with it..... That's world class, right there!!
I fully intend to purchase one of their conversion kits for my '93, as it's original transmission is having difficulties. Let us know what you decide, and how it goes, please!!