Well, based on your goals, I would say stick with the 250 engine, all the best features of a 200, 7 mains, etc... carb swap(direct mount for more power, or adapter with a H/W for mileage/easy install), cam swap or increased ratio rockers for a little more go. If money allows, aluminum head would be great as it flows much better than stock.
Fuel injection would be a great addition, but that is a bit too involved for me right now.
With the right ratio rear, efficient tune and all, mid to high 20's are possible.
Be Blessed and have fun with the project,
Ben
P.S. do some searching on the mod's you want to do, just about everything that you need to know is in the archives, if you can't find it, ask, the guys here are the best source of info that I have found... even when you ask a question about the bent-8 motors...