Externally the blocks for the 1956-61 250, 63-66 287, and 1957-66 327 are identical. Cranks and rods interchange. The only difference between the three is the bore in 1/4" increments -- 250 has 3.50" bore, 287 3.75", and 327 4.00". All other parts interchange -- I don't think the 327 even uses bigger valves than the 250. The only difference other than the bore is that the 250 used a solid lifter cam and adjustable rocker arms (and even those will interchange with the other motors, and the 250 can have hydraulic lifters and rockers installed).
If he can find that casting number (should be on one side, have 7 digits and start with a "31" -- there are two 327 numbers that start with a "4" though, and one of those may only have six digits) I sure would like to see it! It may be a 327 casting number.
I just want to reiterate (for other readers) that only the EARLY 287 blocks are supposedly thick cylinder wall 327 blocks (which would have been cast in 1963, March-June, most likely would be in a late 1963 model car, the 287 was introduced in May 1963). The casting number may be the same for all whether the cylinder walls are thick or not. I haven't seen many 287s nor had any checked, I'm going by second hand info from those who have. Most 287 blocks are not thick enough to take a 1/4" bore. If you have a 287 the date code is on the generator/alternator bracket, stamped on a tag that is screwed on. AMC did all their V-8s that way (later ones have the tag on the front of the right valve cover), they didn't stamp a date code on the block. That's the one thing I really hate about the V-8s! So check the cylinder wall thickness before boring.