is the 600 and 650 a big jump in gas difference?

plus i will need some different valve cover i think. the stock one's on it now has an open port coming off of the oil input neck.
and i was told by a friend who works at the advance auto parts by me that there might be something up with the breaks lossing presure. one good thing about having friends in parts stores who deal with everyone in the area. they know whats up with all the cars in the area, lol.

it's already starting to sound like the car is to much for whats it's worth but i cant pass up an even trade for it. mine's only a 4 banger, a clean and well tuned 4 banger but im sure you know what i mean.