Congrats! Keep looking for rims,use the tires. That thread on the longer shocks with S/S springs, is good info.(Use a basic 18-22 dollar O Reilly's/Auto zone rear gas shock. Order for a 70-71 Chrysler Imperial)
In the link on SS springs they mention using a 68 Imperial shock on the back. Do you think there is any difference between the 68 and the 70-71 shocks? I am having better luck finding wheels with either 5.125" backspace or 5.50" backspace. The 5.50" backspace wheel would require a .250" thick spacer. The 5.125" wheel will have the tire sidewall close to the fender lip, but some careful trimming could provide the needed space. It is just hard to spend $109 for a steel rim.