If milage is more important than torque build the 250 you already have. It will be easier to get 20s from it than a 292. I don't know what the first and second gear ratios are in your 3 speed and how they would work with the Jag rear end ratio. To get good milage the engine can not be working hard at your cruising speed. Look at the post by Tom Lowe about his recent trips in his 250 powered wagon. Remember that any money you put into the head, headers, intake, cam (with electric fuel pump), and ignition won't be lost if you later decide to go to a 292.
