I agree stainless is the way to go. If I was going your route, I might consider temporarily remove the composite pad (if you have one) and trace the hole locations onto something (possibly a big sheet of cardboard) and use that as a template. Your margin for error is Slim to None (and Slim left town.) :) I might even try to figure out a way to use the composite pad itself as a template.
Don't forget to use anti-seize compound on the stainless to prevent galling and, from what I was told by the bolt manufacturer, possibly breaking the bolt when tightening.
I used wedge anchors after the concrete set up. We positioned the composite pad, marked the holes, removed the composite pad, drilled the holes, inserted the anchors, put the pad over the anchors, then lowered the generator onto the pad (lining up the holes.) The crane on the propane truck made moving the generator easy. Everyone won't have this luxury though.
5875, Nexus controller, 999cc Engine, 20kW LP, 2011