The 400 and 455 are externally the same size, but very different internally- the 400 has 3.0" mains, and the 455 has 3.25" mains.
You can still turn a 400 into a 455 by getting an Eagle cast stroker crank, forged I-beam or H-beam rods, and new forged pistons. I don't recommend reusing the stock cast rods since a new set of forged rods is only about $100 more, and the stock 455 pistons have a different deck height, so you need new ones. This kind of stroker conversion will cost about $1500-2500, depending on the kit you buy and where you buy it.
I've been doing some comparison shopping- you can actually buy all of the components of a 400 to 467 stroker kit separately, and it will cost about $1100-1200, alot cheaper than buying the kit.
As far as finding a running 455, look in the local classifieds for '70-'76 Pontiac full size cars and wagons- you can buy these for a reasonable price, pull the motor and have the rest towed off. The boneyards may have them too, but will charge you more for them. I personally recommend building the new motor, so that you can learn and get a fresh motor that should last for a long time.
I 'd get a new block, 400 or 455 (whichever is easier and cheaper to get), decide what CID I want, and then build it. The 400 in your G/P runs, so no worries there- then when you're done with the 455/468, swap it in and you have aspare motor just in case.
I like the 400/468 stroker kit w/ the cast Eagle crank, forged 6.8 4340 H-beam rods, Ross pistons, and bearings because it's a good value (most speed shops on Ebay are selling this combo for about $1400-1500).Run good D-port 72cc heads and make about 475+ hp, or break open the piggy banlk, run E-heads and make 550+ hp. Good times!