If it took a 1/4" overbore you're lucky! It's been RUMORED that the early 287s were actually cast with real thick cylinder walls, but not substantiated. That's a LOT of extra cast iron when an easy change to the sand cores would allow for a thinner casting. As long as it works you're good, of course! I would just go bore a 287 out to 4.00" (stock 327 bore) without sonic checking the bores to make sure they are indeed that thick. I'm sure your machinist checked it first. Stock 287 bore is 3.75".
I wouldn't mind knowing the casting number on that block. The only casting number I have for a 287 is 3169824. I have 8 different numbers for the 327, only two for 63-66 though. Would be real interesting if the casting number were for the 327. The bore should be cast into the block on top just behind the right head.