Hi Dave,
Your best option may be a T18/T19. More info here; https://www.novak-adapt.com/knowledge/transmissions/manual/t18-t19.html
They are short, durable, and parts are readily available including complete units. A complete parts list for the conversions is below.
8133951-K, Bellhousing
TR-T18, Transmission. Can change to TR-T19 depending on your preference
183-F, Trans to t-case adapter
RMU, Transmission mount
PB-51881176, Pilot bushing
TB2, Throw-out bearing
01-015, Clutch kit
F-7TTC, Transmission tunnel cover
300MR-2, Dana 300 rebuild kit (optional)
8131657-K, Dana 300 front output rear bearing cap (upgrade)
D328-S, Dana 300 oil pan (upgrade)
Let us know if you have any other questions or would like help ordering.