I've always liked the 59L blocks - my first race engine came from Kenz-Leslie out of Denver and was a 59L. The relief is obviously a factory relief - which was typically done on trucks and other severe duty applications.
My guess is that when it was rebuilt, the rebuilder bored/sleeved it back to STD. Ford would also bore blocks and sleeve them to STD to save a block. I'd measure the sleeve thickness and determine what the bore would be if the sleeves were removed. What you don't know is if any of the cylinders have issues (underneath the sleeves) - you'll find out!
If it is under 3 5/16, then boring it to 3 5/16 would be my "goal" (depending on a mag test, pressure test and sonic test).
As noted, building a really nice 276 street engine would be great combination and give you a lot of torque and performance to boot. Best of luck!