I recently experienced a two tire (245/40) blowout (both driver side) and both 18" rims were cracked beyond repair. I had 2 17" rims sitting around with tires (245/45) so I decided to temporarily put the 17s on the front while the remaining two good 18" wheels are on the back. Is this going to cause problems if I keep it this way?

 

no problem there. a lot of people run front and rear staggered setups. you should be ok

 

If you were to run different sized rims front and back you would need to have the appropriate sized tires on them so their over all diameter would be the same.

From what I remember you should be ok with those sizes, both of your wheels should have a total height of 25.7

 

should be fine. Corvettes use this setup, usually it's 18 front and 20 rear. Do you have a picture? It would be interesting to see this kind of setup on the SC.

 

The next time I go for a drive I'll take a pic.