Physically for MC I see Matt Damon bearing a pretty strong resemblance. He has blue eyes, brown hair, he's white. He's 40 years old irl but looks closer to 30, which fits the S-IIs quite well, being 41 but certainly looking younger than that. Though he's a few inches short, I think that is easily overlooked given that by the time MC is fully grown he's in mjolnir armor anyways.
For his voice no other than Steve Downes, but for the sake of a movie I have no problems with Matt Damon voicing him. I think back to his experience as Jason Bourne and such, his less childish roles. While they can obviously do voice overs, it would just be weird hearing downes' voice coming out of damons body.
Linda is a redhead (my favorite) so to get someone befitting of a slightly more nmature but certainly not remotely old look, Christina Hendricks. But I mostly say her because she is beautiful.
[Edited on 03.09.2011 11:50 PM PST]