I think this is n excellent idea, personally. Either one will do what you want, with a 20" barrel or less. Velocity loss will not be as big an issue as muzzle blast.
The 450 Marlin is, as you have read, an easy swap, and some minor "adjustments" to the magazine might be necessary to open it up for the wider round.
Yes, all you'll need is the bolt head, another beauty of a Savage. I did exactly what you're thinking of doing with a 30 yr old 243 I had. I went with the 308 because I live in CA where lead hunting ammo is banned, and 35 cal lead free choices are very limited at best. The 358 was my preference.
Here's mine.

Prettied the stock up with a forend tip and new finish, and barrel cut to 20" and recrowned. But if she gets scratched, she gets scratched. It has a 4X scope on top and might end up with a 2.5. I kept 4X for now cause I can easily got from brush bustin to a 250yd shot across a canyon.
I'd suggest the 358 cause it will be easier, if necessary, for a followup shop than with the 450. The Marlin round can be loaded lower, however and I'd really recommend doing so. All the power of the factory 450 is not needed unless you're after the big bears. BUT, the cool factor is pretty high.
Ammo cost will be a lot higher if you do not reload. Availability will most likely be about the same, with a slight advantage to the 358, be neither are common rounds. The brass for the 358 will be easy, buy 308 and neck up.
To me, though a light gun is also a brush gun, and light gun and 450 Marlin is not a good mix.
Have fun.