Given what you have available to you, I think my first choice would be split omni's on-stage. Even though you said you don't have anything to mount them onto, it seems simple enough to jury rig something up to hang each mic a couple feet up off the stage. Just try to make sure each mic is positioned logically so that no single instrument will overpower and that to whatever degree of control you have, that all instruments on that side of the stage would have similar volume at each mic. Obviously, you wouldn't want one of the mics 2 feet from the kickdrum, for example.
Second choice would be to run your cards DIN (90degrees, 20cm apart) near the front of the stage. I'll assume that you'll be singing vocals from stage front, so perhaps you could position the cards just below the vocal mic at the stage lip (but behind the on-stage monitors) so that you won't be picking up much of the stage vocals (since you said you'll be mixing vocals in from the SBD). I'd go DIN because if you go wider angles, you might lose whatever is in the center. I'd also try to make sure that the on-stage amps aren't the outside of the DIN angle. IOW, wherever the mics are setup, make sure that the angle of incidence of the mics doesn't exclude any of the on-stage sound sources, so before the show, move your amps towards the center if you need to in order to keep them from being too far outside the DIN angle.
EDIT to add: You might even consider NOS spacing to enhance stereo imaging. I know people that have had great luck with NOS spacing near the stage lip. I've personally had great luck with imaging in the DIN config at the stage lip and I'd be concerned with too much spacing and losing the sounds from the middle, but NOS is worth considering. Having said this, my favorite config at stage lip is mid-side, but that's not an option for you.
Both of the above options should give you a nice stereo recording of the instruments and then you can mix your vocals in to taste during post. The first option would potentially have less crowd noise, unless there's some way you can keep people off the stage lip yelling 'free bird' at you.
