My crank is at .030 now and needs to be machined again. I have another thats already at .020 but it's pretty rough. Is .040 ok for less than 500hp? Or keep looking for another crank?
A -0.040" undersize crankshaft for a 500 hp engine is fine. And it might even be debated to be technically better suited than a standard/standard crankshaft, but that's really splitting hairs and in reality one is not necessarily better than the other except for the amount of the crankshaft's remaining service life (future regrinds).
Don't worry that it's -0.040" undersize; regardless of journal size
any crankshaft has to come out again if it gets damaged so it isn't important whether it's standard or -0.040". If you can get bearings for it, a freshly turned crankshaft may be considered a "good-as-new" crank in a case such as yours, and will last as long as a standard/standard crankshaft in your bulid.
Paul