EDIT: nvm i see you gotta do 2 of them for 2nd and 4thThe Sonnax Servos are a great choice over the corvette Servo and you should have no issues or problems.. check out these links and Part Summary for more info to help with your decision.
2nd Gear "Super Hold" Servo
4th Gear "Super Hold" Dual Servo
Remember you always get what you pay for with aftermarket parts. People commonly mistake the design flaws of the fairbanks servo to the sonnax just because they are both made of "billet". However, they could not be more wrong.i guess once i figure what price difference is go from there. is the sonnax gonna be rougher on the trans causing a higher rate of failure?