'71 LeMans?
Meh, if it were me I'd go ahead pull the 350 and probably a TH350? replace that will a built LS-series engine and 4L60e or T56
While it was apart I'd also go ahead and do the other "chassis" mods you're talking about.
But I'd go with 15x10 in the rear*if they would fit* and 15x8 in the front.
But that's just me, I like the FI LS engines, and I think 15's look better on old cars...
But just upgrading the 350 in it with a 4bbl & a cam would help it.
and LT's are a must if the application supports them.
Maybe upgrade the tranny in it? If it's a TH350 get a 700r4/4l60, overdrive and lockup TC will help a lot with fuel mileage if you drive it a lot.