There shouldn't be any problems fitting most 16" aluminum rims on the 8-lug stuff. I've seen lots of 3/4 and 1 ton trucks running around with this setup. I have personally had a couple different sets of 16's on a D60, and though they were steel the D60 has bigger brakes than the 3/4 ton stuff.
Good luck finding a set of aluminum 15" 8-lug rims. The only ones I have ever seen were Welds and that was back in the '80's.
Steel 15" 8-lug rims are very easy to find but like most steel wheels there are not a whole lot of styles to choose from. Pull up 15" 8-lug wheels on Summit Racing and there are 45 versions pop up. 4 Wheel Parts and National Tire & Wheel also advertise them on their website, and you can order them through most tire shops.
But back to the OP, the 16's you mention should not be a problem.