Yes you can swap over 243 heads. BUT...... you need to measure for push rod length. Especially if you take heads to a machine shop and have them true up the surface. Highly recommend this if you get a used set. It's not hard to do. There's even a how to help thread in the Tech sticky threads.
:lmao: I wish I got 17mph.
I'm also Speed Denisty tuned.
Are there any changes to the rear gear ratio?
if auto did, you change the stall to deal with the new engine. iirc the cam profile affects the torque curve, and that will also affect mpg as well.