It's all about the name. You see, back about 20 years ago, guys would take the 440 in thier car to a machine shop, and have it punched .060 over. It results in 451.something cubic inches. But it was "a 440 sixty over...". What that is very simply is an RB block, with a max overbore. It works, but there's been some newer thinking (more because of sonic testing now) that showed many blocks are not terribly thick where it counts, and that's what keeps the rings sealing...the lack of movement in the cylinder walls themselves. So, .060 over is more of a "sonic test it first" thing now in many shops.
Now, the "451" you keep reading about, is a B block based engine. It's smaller, lighter, and more compact. The 400 B block has a 4.350 std bore. 440 RBs are 4.325. So it's already .025 oversize. Bore it .030 over, and run the RB crank, and you get 451.something inches. Plus, the longer stroke, and longer rod of the RB. It's a win/win engine really. There's a ton of good and medium price group parts available now for them too. The first one I built in 1993 cost me $450 in crank work, and the pistons were $800 J&Es. The last one I built last summer, the 440Source crank, rods, pistons, bearings, and rings were $1400. The cranksare much better, the rods much better, and the pistons a little worse than what I built. But WELL worth the money in an A body chassis. Especially with better heads. The B block is narrower, so changing plugs and header fitment is easier too.