What Metals are Safe to Have in a Case?

I'd say any except magnetic.
None of your components are really going to be touching the case.
If you could elaborate on the modifications that might help us give better advice.

polonium, mercury, lead, cadmium, vanadium, uranium, cobalt60 just to name a few........... get a hair net.

Just making sort of a backplate next to the motherboard and underneath the DVD drive so I can fashion 'on their side' mounts for my SSD's.
Would aluminum work?

Just to be safe, go with Wood.  

Just make sure nothing is touching the mobo or any exposed boards. Basically just so you don't short circuit something and you should be good  .

Thanks  
One last thing, what ~12"x12" cheap metal sheets easily available at say Home Depot, Rona, Lowes etc. would you recommend?

Steel or aluminum.. That's what I would go with. No clue whatsoever on the price thou.. I'm sure a quick search online can give you a idea.. But can't be to expensive for what your going to do.

Do us a favor and post some pics when your done! Always like to see DIY projects before and after  .

Thank you very much for all the help. I've been searching said stores' online warehouses and such and I haven't found any aluminum sheets o:
I'll check steel afterwards.

The plan involves a tad more than just the SSD mounts, I'll be documenting it via pictures and video for YouTube and put the link in this thread when it's done. I was just going over all the materials I'll need before starting and came to this little predicament, lol.