Torq Thrust wheel sizing

Currently running 15" cragars on my 73 Duster. Not sure if they are 15x7 all of the way around, or 15X8 on rears. Front tires currently are 215/60/15 and 255/60/15 on rear. Looking to go to traditional torq thrusts instead for wheels. Should I use 15x6 or 15x7 all of the way around or use 15x7 in front and 15x8.5 in rear? 3.75" backspace? I plan to use the existing front and rear tires.