Retaining 30-35 mpg shouldn't be a problem. But you need to answer 2 questions: how much money do you have to spend, and are you doing the work or having someone else do it?

 

A very nice touring combination that we use here is as follows;
Our Stage I Evo headwork
Wiesco pistons
Wood 6 cam
S&S pushrods
Dyna ignition
80 hp and 90 ft/lbs is what that does.
A Long Island N.Y., based shop, recently tuned 84 hp, and 93 ft/lbs with this combination of ours.
Fuel economy should actually increase.
Scott

 

go 106",4.625 stroke with 3.12 bore,1.900 headwork,woods W6H cam.we`ve done many touring evo`s with this build,very reliable & a real torque monster!!

 

Another option, to stay with your existing 4.250" crank, is an Axtell 97" cylinder kit, combined with a Wood 8, and a set of good heads.
Reliable, cool running fun, that'll deliver great fuel economy as well.
Scott