Question; Why do you want to go to the later suspension? Are you going to bag it?-KB
Unless you replace the whole rear frame section,I don't know how that would work. The 70 rear section has a hump over the axle and the 55 is flat. Doesn't look like you'd have a lot of room for springs. The way the trailing arms are set up,using that suspension ,I think that your pinion angle would be way off,which would require moving spring pads or using a tapered block to correct.Anything can be done but I think that you will have go through a lot to get that newer stuff to work-JMO-KB