295's should fit easily on your 10's (I know guys that have 275's on 8in!), thats what I would go for! BUT dont forget to check and measure your back spacing from your standard wheels and tell the Rim guys so that the tyres dont rub on the trail arm and park brake Cable. You dont want them guessing, ask them if they know what the backspace for the C3 vett is or get them to measure it, that way its there fault if its wrong!!! Its probably on the web somewhere anyway? *this is important before ordering custom wheels!!!. Also ask for the SOFT LIP it looks better than step lip! It looks FATTER
What series profile tyre will you go? 50 series should do it,otherwise you'll have to jack the back up! 255/60/15 are 27in tall, 295/50/15 are 26.6in tall and 295/60/15 are 28.9in tall so it would look silly!
I bought Offset trailarms to get the extra width! (12 inch wide RIMS) and asked somebody with similar wheels in the US what there back space was!
Good Luck