drill a new hole in the carb arm but, match the distance from the servo arm .
if the distance from the pivot point on the servo arm to the hole you are useing is half inch then
drill a new hole half an inch from the center of the pivot point on the carb arm .viola! linear travel!
I like longer arms on the carb. My servos and radio will allow 120 of more degrees of rotation, most of my Walbro carb's throttle shafts rotate less than 90 degrees maybe 75 degrees. With equal length control and servo arms I can not use more than about (75/140) 53% of my radio's adjustment ability. If I use a short, say 1/2" servo arm and a 1"+ arm on the carb I seem to have much more adjustment on the radio.
I would be concerned about is using solder as the only mechanical link to 2 critical parts. I was always taught that solder is just a way to ensure the mechanical connection doesn't come apart.
Good point. I'll consider drilling a couple of holes thru arm into collar and putting a couple of pins. Vibration more than to shear loading on solder joint.