I use tape on my rails you should be fine,did you do both sides
,as regards ther radio box they can flex a bit so a good idea is to use some plywood in the base to fix your servos to so the box doesnt flex etc.Not familiar with that box it might allready have plywood insert?.Personally i make my own wooden radio boxes as i find them stronger,but lots use your type so you should be fine.With that pipe on that header you wont get the motor to rev out as it will be too long and you wont be happy with the performance,i think as said you should either send it back and ask to swap for a 105% header or sell it and buy one just my opinion.1" or more too long and you will notice a massive difference than having the pipe set at the correct length,on my hydro i had the pipe set too long by 1 1/2" due to a mixup and speed was way down maybe 40 mph but once i put it right it was like night and day and i picked up like 15-20 mph so it does make a big differnece.
Mart