M20 was a side shift, I didn't think it was an option in 1980, plus I don't think it was able to drive a PTO. What you likely have is the SM465. This had dual pto covers top shift, and cast iron case.
The 2nd nv4500 is about 8+ inches(I think) longer then the standard 2wd sm465. The bellhousing will not correctly bolt up, you would likely need to get the Advance Adapter bellhousing. 80 should still be mechanical clutch. You could convert to hydraulic clutch ND use the nv4500 bellhousing and hydraulic slave.
That's pretty much the basics.
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