first I wouldn't use JB Weld soldering is easy it just takes a little practice, with the servo arms, I usually junk them when I do the"through the servo mount plate installation and replace them with Dubro HD servo arms http://www3.towerhobbies.com/cgi-bin...?&I=LXD668&P=7 there thicker and slightly longer than the stock futaba arms and there will be plenty of plastic when drilling out for 4-40 rods.

a alternative to soldering would be to use a standard pushrod with a clevis screwed onto one end and make a Z bend or use a quick link after making a single bend using these http://www3.towerhobbies.com/cgi-bin...?&I=LXEMG3&P=7 I use them all the time they work great and for added security you just slip some fuel tubing over the end snapped onto the pushrod and it will last for a long time.

as for the mount, your better off making the mounting holes to the firewall larger, than shaving any material off the beams

if you can not move the mounting bolt locations,make sure if you open up the holes in the back of the mount you use a larger washer to spread the load since the bolt pattern will be more narrow