First of all, welcome to the board! Post some pics of your 68 when you can.
Check
THIS out. Travis (ProPerformance) was very helpful with placing my wheel order. Also, there are several (probably hundreds) threads and discussions on the subject. So many people have different ideas of the perfect offset, but there are a lot of common offsets, it just depends on what size you're wanting. I've seen a lot of guys run 5.25" BS on a 8.5" front wheel, and IIRC, 5.5" on a 10" rear wheel. I can't remember if the rear wheel is for the wider rearend or the narrower rearend. It would also make a difference if you have drum or disc brakes. If disc, is it a 71-72 swap, 73+ swap, or aftermarket? All of these things make a difference in what the ideal backspacing would be. Your best bet would be to educate yourself on how to measure, and ask questions of the guys you're going to order from. Measure you're vehicle, don't just trust others suggestions. That's a lot of money to spend to not be sure they will fit.
GOOD LUCK!!!