Replace sm465 with a different type

Your post has hurt my heart. That SM465/NP205 combo is my favorite! You may source an NV4500 from a newer OBS truck & put an NP208 behind it but I don't know about the particulars as I've not done one of those swaps. Maybe someone else here will have some better advice.

How do you intend to use the truck? Daily driver? Mud bogs? You gonna take it up the Rubicon?

When you request options that “will hold up” it leaves room for interpretation.

I don’t know of any 4 or 5 speed available that will be as simple or tough as the 465/205 combo you have.

I ran the “weak” 5 speed getrag 1500 (aka HM290) for a few years in my k10 and it held up fine for general daily driving including some basic off-roading and Sierra Nevada snowstorms.

The NV4500 @Strick recommended or even the NV3500 are both decent.

All of the modern options will require conversion to hydraulic clutch cylinders, moving the cross member, a 208/241 TC, appropriate shifters, moving the holes in the doghouse for shifters, and changing both drive shaft lengths.

Automatic transmission swap is a whole other set of tasks but may be a viable option.

You should start be saying why you hate the 465. Why do you want a close ratio 4 speed? Why not a 5 speed? My NV4500 will stand up to anything out there as far as durability but it shifts like a dump truck. You won't be slamming gears and looking cool but it does shift like butter and has OD.