I have the Extreme energy 274 and it is sooooo mean. No one believes me when I tell them it's a 289. I've had a lot of head work done to some windsor Jr heads and went full roller.
I kind of agree with what was said earlier if you're going for gas mileage and want to stay with the stock heads I wouldn't put in the work to swap the cam.
However, you can find some iron heads like I did, possibly do some flow work to them, and go with a more radical cam like the 262 mentioned which would result in significant power increase.
Although it's tough to wait sometimes it's best to put this project together a piece at a time. Find some iron heads and put them on the shelf until you get your cam and lifters.
Oh and IMO, I would go with solid lifters if this isn't a daily driver and roller if it is. Spend the money to get everything externally balanced before you re-assemble.
Good Luck!