Your proposed setup is very Benoit-y, and I am appreciating your thought process. It is an excellent one for the big northern woods, tried and tested. I too have taken this path...more than once, although not exactly with the 35 Whelen caliber.
Couple things I have learned:
1) What to look for in a buttstock - the 20 ga buttstocks for the 870 fit these, and have the desirable drop.
2) I find the Williams sights to be a bit too refined, made of light aluminum and little biddy screws that get lost easily. Not complaining, mind you, but prefer the Skinners which are made of sturdier stuff.
3) I like the winged rear sight Skinners, btw, they just seem to shed undergrowth from around the aperture in the NEK where I hunt, and if they take a fall - especially in the granite rocks that rumor has God spent the seventh day of creation throwing at Vermont, I am more confident they'll survive. The XS also are good, and I've tried one set of those (Front and rear) but have reverted to the Skinners.
4) On the front sight, it is likely you will need to change the height on the blade when you install a rear aperture. And even when you get the blade you may well have to take it down with a file as you sight it in. Others come with a narrow shank topped by a bead and are not susceptible to filing down to fit. If you don't order exactly the right height you may waste time and money curing that problem. The issue here, again, is sturdiness. Haven't measured it but the Skinner partridge front sight looks to me to be about double the width of what the others are. (Not sure if I think the Skinner winged front sight isn't too much of a good thing, but have never tested that).
5) Paint on the sloping edge of the thick skinner partridge sight really pops out. Kind of like "heavy duplex" reticule on a scope. Orange being better than white for contrast against a snowy backdrop.
Anyway these are things I have tried and tested. Hope it helps.
