I had similar thoughts/goals when I built my .358. I took a Winchester m70 stainless in .243 and sent it to JES to be re-bored and chambered in 358, and cut to 20". No need to worry about seating bullets out further, it can shoot 300 yards just fine as is (I've killed deer with mine out to 275). If you desire more than a standard length 358 can give, just go 35 when for the easy button.
If doing it over, I would just stick with 308. Less recoil, flatter shooting, easier to source both ammo and components, and honestly kills the same. Maybe even better, since there is a plethora of quality .308 bullets, compared to .358 bullets. Anyone want to buy a m70 in 358? Lol