$5000 sounds too high, IMO. For the $12,00 that it would cost, you can buy a really nice one all ready to go. I like the 68s, too, but keep in mind there are a LOT more parts available for the 70-72 models. 72 ragtops seem to be plentiful. Here is a nice car for $12,500;
http://www.cars-on-line.com/32344.html
Here is a list to look at, give you an idea of what's out there, price-wise;
www.cars-on-line.com/oldsmobile.html