The problem with modifying most new production tube amps is that they're made for economy of manufacture, not serviceability. That's yet another reason why so many people are willing to go through the time and effort to build their own using "primative" methods like eyelets or turrets. Printed circuit boards are very easy to manufacture, but a real PITA to service. For example, there are some experienced amp techs on this forum who have anxiety attacks when they see Classic 30's.
Anyway, it sounds like you're on the right track with a 5E3 kit. There are several reputable suppliers of Fender style kits. A few that come to mind are:
Mission Amps - Great kits, reasonable prices
http://www.missionamps.com/5E3kit.shtml
Allen Amps - A little pricier, but still good stuff
http://www.allenamps.com/kits.php
Mojo Amps - Looks expensive, but you get everything
http://www.mojomusicalsupply.com/item.a ... id=5550002
Good luck and welcome to our opium den.