Buy a Rocket III (or a Thunderbird Storm).
Sorry to my America and Speedmaster brethren (and sistren?) but by the time you upgrade one of these bikes to meet your specs you'll be at the Rocket III price (less for a used one) anyway. Big bore, cams, flatside carbs, maybe even a stroker crank... and that's on top of the aftermarket exhaust you'll want anyway.
I love my Americas. They run great (well, one of them RAN great), handle awesome, have tons of character, look beautiful, and they're a rolling conversation piece. But they aren't high performance machines. Some will disagree. That's fine, but I'm being realistic here.
Your bike has mid-40 HP at the rear wheel. 62HP is at the crank. You can get 62 at the rear wheel if you do the K&N pods and aftermarket exhaust. Look at the dyno sheet thread here in the Performance forum.
My second America has the stock airbox drilled with a unifilter and the snorkel removed. The mufflers are long TORs. It will cruise ALL DAY at 90 mph with my rather considerable 300+ ass on it and loaded down with saddlebags and luggage. If I go easy on it it will push 50mpg.
