A 1970 vintage Rem 870 will probably not have the light contour barrel and should be just fine with 2-3/4" steel loads as long as the choke is not too tight. Opening the choke or having the barrel tubed is a perfect solution, although 870 barrels are so ubiquitous that a new/used barrel may be cheaper than having the original barrel tubed.
FWIW, Remington used to have some fine print in their catalog that indicated that standard (non-magnum) 1100 receivers would work with 3" steel shells as long as one fitted a 3" chambered barrel to the old frame. I bought such a barrel "just because" but have never used it. Maybe later.
