Work Truck motor choices? (no big blocks)

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I have a 1958 D-100 that I am restoring to "use" to carry a camper and pull a boat. I am on a budget (generous). I want to still have a heavy duty 4 speed but may go for one that has sinchros for second so the wife will drive it. The truck came from the factory with a flathead 6 in it and now has a 225 slant 6. I don't want to put a big block in it but want to have enough power to pull the mountains without too much of a drop in speed. Here are the choices I have been thinking of.

318 poly- I am thinking I will have quite a wad of cash into it to get the kind of performance I am looking for but like the idea of having a motor that the truck could have been bought with

340- I have always liked this engine and know it can perform.

360- This looks to be what I want as far as performance and has great aftermarket support.

I would like some recommendations as to what engine you would choose and reasons. once I decide what engine I will have in the truck I will be looking for how it should be built.

Thanks, Chris