I've put my engine back together but the crankshaft moves back and forth about 1/8 of an inch. My motor is a 400 chev
It moves back and forth in what direction? Forward/aft of the block? Or side to side? If side to side, are you sure you don't have a 350 crank in the block? The main journals for a 400 are larger than the 350 (400 has a longer stroke as well). The difference in journal sizes is a little bit more than 1/8" (.200" actually). A medium journal should mic to 2.45", while the 400 large journal is 2.65". That could easily account for the slop you're talking about.
Also consider a 383 stroker crank is the same stroke as a 400 (3.750"), but with the medium journals. You punch your 350 block out to 4.030" (+.030 over) and throw the stroker crank in there and you get a 383. If you got the 383 crank, it would have the right stroke, but would have slop in the mains.
I'm pretty sure you can probably get bearings for the smaller 350 journal sizes which will fit in the 400 block. This would be a lot cheaper route than trying to source a 400 crank, as well as it being a lot easier.