what do you think the optimal barrel lengths and twist would be for the .45-70 and the .358 Winchester?
Can't offer experiences with the 45-70; don't own one.
The wife's BLR, rebarreled by Bill Leeper from .308 to .358 years ago, wears a 24" barrel. Bill offered to leave it "long" and if she/we didn't like it, then he would cut it back. I compared the feel and handling of it side to side to an unmodified BLR, and I actually preferred the slightly barrel heavy feel of the 24" bbl. That might be just me, because I have a few Husquvarna Husky's in the safe, some with the original short 20.5" bbl and a couple that Bill rebarreled at 24". I like the slight barrel heavy feel on those as well.
As for my wife, she's a hunter and shooter from way back, but a rifle is a rifle is a rifle to her; the tool to do the job. It's over and under shotguns that get her interest in lines, feel, etc. She says it's because you carry a hunting rifle slung over your shoulder for days and days all year and then shoot it once or twice, but you carry a shotgun in your hands behind the dogs hunting birds and shoot a lot. Anyways, her opinion on the two different BLR barrel lengths was she had no preference whatsoever. So the BLR retained the original 24" barrel length Bill left it at as she didn't care and I preferred it.
We hunt in some pretty thick alder jungles in some of the places we hunt (and where the grumbly bears live, thus the fondness for .35 caliber rifles). just don't find an extra 4" (or 6") of metal tube is a hindrance in either carrying or shooting in that cover. If you're doing dynamic shooting between multiple targets, fast shooting at grouse in heavy cover, etc., then I think most people will find they have more success with shorter bbls. But not for the essentially static shooting that is the case in probably 99% of shooting at big game.
And while many people don't think the .358 Winchester is a "ballistic marvel", it depends on whether you want to stick with the anemic offerings of factory ammunition or handload for it. If you are going to be in heavy brush always, then you're probably thinking of really heavy bullets to hopefully reduce deflection if the bullet hits some brush on the way to the critter you're shooting at, and ballistics really doesn't matter. I load the old original Barnes (I think - I bought a whole bunch of these when the store I bought my 358 Norma Magnum from way back offered me a deal as they weren't selling) 250 grain round nose for both her .358 Win and my rifle when we're in the heavy stuff. POI is not exactly POA with those bullets, but at close range in the alders, if you're worried about that, you probably look for fly shyte in your pepper.
The everyday load for the wife's .358 Win is the Barnes 180 grainer. It will easily ballistically match a bullet of the same weight out of a 30/06 to a little over 300 yards - and nobody says the 30/06 is "just a woods rifle", or that the 180 grain bullet in the 30/06 is too light for elk, moose, bears, etc. I understand some people say they have issues with Barnes' performance, but I've been hunting with them in some applications from before even the X-bullet was first announced, and transitioned from the Nosler Partition to Barnes for all my big game shortly after that. Deer, elk, moose, two goats, antelope... always performed as advertised.
BTW, my wife shoots hundreds of commercially cast .358 158 gr. SWC pistol bullets every year over a little bit of Red Dot for about 950 fps - likes to "heavy up" for those big bull gophers. She'd rather shoot them with her BLR than her CIL/Anschutz Model 190 - go figger. I got sick of cranking them out for her, and she finally turned into a reloader.
For your amusement, this is an old article by Paco Kelly regarding the possibilities with the .358 Winchester. He's been a rabid experimenter and gun writer since before the Web existed.
http://leverguns.com/articles/paco/358_wcf.htm
Another one I stumbled over while looking for that one for you.
http://gunblast.com/Browning-BLR358.htm
What do you intend to hunt with your 358?