some thoughts before you pour:
Tom has an elevated chunk of concrete, getting the engine up to a easy access height (sorry - didn't see the photos at first)
Will that form bulge, will it need some 2x4 mid way to brace it ?
Are you bolting right into the concrete ? Where do the flywheels go ?
How does the mounting of the generator head relate to the engine - will they be on a frame, or bolted to concrete ?
I drilled into the concrete and used structural epoxy to anchor all-thread, provides a little bit of shock buffer, it's possible for vibrating threads to chew up the concrete. And I've heard you get a stress riser in the 1/8" of bolt between the concrete and mounting nut, that short piece of bolt has to handle all the load and can get really stressed. (use s small dowel to cast a pilot hole)
Lots of rebar right ? and a high strength mix, not post hole mix.