The 7 1/4" WILL fail behind a daily driver 318, it's just a matter of time. Broke more than one myself. The 8 1/4" WILL fail behind a street/strip 340. Ditto.
You can break anything if you try but considering that millions of 318 A bodies were made and the stock rear end was a 7.25 and they held up fine in normal daily driving conditions. The 73 Swinger I picked up from a guy at work (he bought new) for the front disk brake parts had over 300,000 miles on it and the rear end never failed according to him.
Same for the 8.25, lots of A-bodies run low 12's and high 11's for years with an 8.25. It ultimately may not be as strong as an 8.75 but it is a stout rear end and is still being used under pick-ups and Jeeps.
Other than the bolt pattern and drive shaft issues mentioned the plane seems perfectly good to me.