72 340 Duster Wheels

Hi all, New to this site and working on an original h code 72 Duster. I want to put the rallye wheels back on the car and I am looking for info. I have the original 14x5.5 wheels but I want more rear tire than these rims will allow. I also noticed that 14 inch trim rings are more expensive than 15 inch. The only rings I see being sold are for 6 inch rims. Will these fit? Whats the best source and price for sets? I may want to put the 14's up front and 15's in the rear.
Did all 340's come standard with the LBP wheels? How about backspacing limitations?