Do you really want the car to be a daily driver? A small block car will get better mileage, handle better, and be easier to work on (more space!) than a big block car.
I'd keep a small block in it. Talk to the guy with the 340, maybe you can talk him down a little. Heck maybe you can work out a partial trade with the 440.
There's a lot of extra expenses with going to a big block. You need a new transmission and driveshaft first off. Then, you'll want to upgrade your suspension for the extra weight and power- bigger torsion bars, sway bars if you don't have them, etc. Depending on how your car is set up, you may want to convert to disk brakes (no mention on your tag if your car has them) or even put bigger drums in the back. An all drum brake A body with a 451 would be pretty interesting to stop.
Even once everything is done, it'll cost more to drive the car- gas, insurance, tires, etc.
I'd sell the 400 and the 440 and buy the 340. Less money to spend on the rest of the car, less money to spend at the pump, and the added bonus of it being a date correct engine for the car. Since you seem to have a pretty highly optioned car, that might actually matter down the road if you sell the car at some point. Also, since the 340 is disassembled, the fact that it's a later "detuned" 340 won't matter. You'll be building it anyway, so you can up the compression when you install new pistons. No reason you couldn't have a 400 hp 340 (or more, but the price starts to go up), and it wouldn't cost anymore to build that than a 451 other than the cost of buying the 340. But even that you could offset some by selling the big blocks.
