The "914" casting is pretty rare. It IS a 400 Pontiac, as well as a BBC. IIRC, 914 is the '70 Ram Air block. If so, yes, it will "fit" in your later F-body. Some had the number fully "cast in". Others had the last three digits ground off and the 914 "stamped" in (factory). There must be 5 mounting holes drilled and tapped on each side of the block (as Lynn said). The date code is on the top by the distributor. Alpha-numeric, 4 digits. It looks like a "tag" but is cast in. The first digit is a letter. The next three, numbers. Same as Chevy and other GM castings. The letter indicates the month (A = Jan, B= Feb, etc.). "I" is not used. The second and third numbers are the day of the month. The last number is the last digit of the year it was cast. Example: A109 would be Jan 10, 1969. It COULD be Jan 10, '59 or '79, but they didn't make the same blocks in those years. Deduction brings you to the correct date. Dates coded after July 7 (or so), are next year's model. In those days, they would shut down during the week of 4th of July to "retool".
If it has only two mounting holes, it's a '67-'69 block. Not all is "lost". Ames Performance sells adapters to use the earlier block in the later body. Not too "hokey", either.
If it is a '70 Ram Air block, get the engine code. Unless you're planning a MONSTER, that block is much more valuable to the collector crowd, and you could sell it, buy a 988 plus a few other goodies. A true "WW" ('70 Ram Air IV, manual trans) block is worth a couple grand if "usable". A "WS" is a popular code as well (Ram Air III, manual trans).
What are your plans? You don't NEED 4-bolt mains until HP passes about 650. Even then, it's debatable. We have many 455s "out there" with 2-bolt caps and factory bolts making 600 and living. 400 blocks are tougher. I like them a LOT better. We have one man with a 455 in a 3,200 lb. GTO going 9.80s with 670 heads (iron, d-port). The thing is a torque ANIMAL. The front wheels come up about 3' when it leaves. He's making more like 700, and probably near that in torque. It cracks a main bulkhead about every 60 passes or so. He considers a "new" block maintainence, not repair. We're starting on a new one using a 400 with "splayed" caps and an Eagle forging. He should be able to make a couple hundred passes before any trouble may start. Going to "more" cam, too. He wants lower 9s.
We (Lynn [LATECH], Bill Creech, myself, and others here) can steer you in the right direction regarding the Pontiac. It's a whole different beast than a Chevy, so bear that in mind when gathering data. We don't "do" them the same as we did 5 years ago, much less 25... 400 Pontiac has proven to be among the best of ALL the muscle car era powerplants. Modern goodies make it run just as "good" as any other older design using modern stuff. And "real" connecting rods are now available cheap. DO NOT consider using the stock rods. Even a 2-bbl. gas mizer should get forged rods...