[Wiking]

I've long thought that Steel Panthers would be well suited to at the very least representation of the Imperial Guard, and perhaps small squads of Space Marines. But it would be an awful lot of sprite work at a minimum, and of course all the LBM's and OOB edits you'd need to do.

All that said, I think the best thing you can do is advocate that the Space Marine player set his infantry toughness to either 150 to 200%, and that will have to suffice. Any other option is not going to be very believable (swarms of light tanks representing marines I think would lead to a host of undesirable behaviors).