Price a Gear Vendors Overdrive... then make an honest list of the parts to install an NV4500 and pad it by another 40% for the stuff you didn't know about.
Having done one I'll tell you right now... the next overdrive manual I'm doing will be a stock granny 4 speed with a Gear Vendors unit. Time and $ ahead to go that direction.
If your truck is 4x4 you don't have to worry about the speedometer. It's driven off the output of the transfer case... That was my solution on the 76.
You'll have to get creative with a 2wd unit. Dodge used cable speedo on the first couple years but GM did not.
There's the Dakota Digital VSS to cable adapter. The Abbot Cable X is similar.
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1959 M35A2 LDT465-1D SOLD
1967 Dodge W200 B383, NP420/NP201 SOLD
1969 Dodge Polara 500 B383, A833 SOLD
1972 Ford F250 FE390, NP435/NP205 SOLD
1976 Chevy K20, 6.5L, NV4500/NP208 SOLD
1986 M1008 CUCV SOLD
2000 GMC C2500, TD6.5L, NV4500
2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263
2009 Impala SS LS4 V8
RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials...
Please include at least the year and model in your threads. It'll be easier to answer your questions.
And please let us know if and how your repairs were successful.