Thanks for sharing a photo of your sharp '71. I can understand keeping her mostly stock. My '70 SS is about 99% stock but my '70 4-speed had such a modified engine when I bought it that I have no problem making the changes I like.
As you probably know, there are some nifty tire size calculators online. One that I like (because it also calculates and compares speed, gear ratios and wheel offset) is tiresize.com. You can first compare all of the key tire dimensions between you 225/70s and any of the 60 series tires. Then, if you scroll down a ways and look on the left side of your screen you will see a list of the other calculators. Click on the Wheel Offset Calculator and it will give you a visual comparison of two different tires mounted on either the same rim (offset = 0) or a rim with a different "offset" or back space.
The dimensional results of your comparison are shown to the right of the visual comparison. Given those changes in tire height, width and position when mounted with no offset, you can then add whatever positive or negative offset (difference between original and new back space dimensions) and it will show you the positional comparison visually. If you are increasing the back space dimension from stock to a new rim, enter the offset amount (difference in back space dimensions) as a positive number. It is really fun to play around with different tire sizes and rim offsets and see how it will affect the new tire position relative to the car's structure around it. Note that you can use this feature for both front and rear tires.
Increasing the rim width from 7" to 8" should not change the sidewall dimension on a 60 series tire but increasing the back space dimension (relative to the back space of the stock rally rims will move the tire sidewall inboard by that amount. It is the combination of a significantly wider tire and increased back space that can create clearance problems on the front. For example, compared to your 225/70 on a stock rim, a 245/60 on a rim with .4" (10mm) greater back space will put the inboard sidewall .74" closer to the inner fender/frame while the outer sidewall remains in almost exactly the same position.
You can see why going with any greater than a 10mm (.4") backspace increase compared to your current rim would be completely unnecessary and could cause rubbing on turns. In fact, you could somewhat "split the difference" of the extra tire width by going with only a 6mm (1/4") positive offset (greater back space) which would only move the outboard sidewall .11" further out than your current tire and the inboard sidewall .59" further inboard giving more clearance for turns.
On the rear, a 255/60 will be .55" further outboard with zero offset so a rim with a backspace at least .5" greater than your current stock rim will be necessary to keep the outboard sidewall in about the same position as your current 225/70 tire.
Net: While greater back space is definitely the way to go for bigger tires on the rear, too much back space increase can cause problems on the front.