I think what MNWild meant was that he needs the car to drive to the parts store.

MNWild - before you remove the drive shafts, be sure to index mark them so that you put them back together in the same relationship to each other. Otherwise you may get them out of "phase" and have some vibration when driving. Some of the 740's are keyed at the center spline (my 1990 was) so that you can only reinstall them in the proper relation, but some may not be keyed so marking it is important. Just to be on the anal safe side I also like to mark the end yokes position relative their respective shaft although many people would probably say that is overkill. It's just that you don't know how the assembly was balanced originally.

Also, before you disassemble it, you may want to shake the shafts around at the center support (one hand grasping each shaft). You should be able to mush the center support rubber somewhat, but you shouldn't be able to here any metal-to-metal banging. If you do, now is the time to replace that also.

If you can jack up the car and safely support it, then you can do this job. Good luck.

BTW, I think I recall doing the same thing - buying both sizes and returning the unused ones.