1965 LeMans Convertible
 

I've had my 65 LeMans convertible for almost 25 years. Two questions:

1. Are there any easy ways to increase the performance of my 326 cu.in. 2 barrel carburetor engine? I wonder how hard would it be for me to have someone exchange it for the GTO 389 cu in engine?

2. The car now has a shimmy around 40 mph which disappears driving a little faster. I've had a good front end guy look it over and all he could suggest was to get new tires. I got new tires but the problem is still there. Any suggestions?