G'day all, heres a little food for thought.
I have taken a 250 X-Flow to 120 thou overstandard. That is from a 3.680 bore to a 3.800 bore. This was done with out sleeving the block. You WILL need a good block though, as the cylinder walls are getting quite thin at this stage (i.e. approx. 120 thou by my thickness tester). We did grout the bottom of the cylinders, as a just in case measure. Interestingly though, I never had any problems with overheating with this motor, which did plenty of varied driving, from peak hour traffic in the middle of Brisbane, to looong stretches of 100 mph+ in outback QLD. If anything, the motor tended to run a smidgen cooler than the previous standard bore motor. This was all in a TE cortina with a standard copper radiator.