There seems to be quite a bit of confusion about swapping to a coil sprung servo. I've recently been through the same process.
You can fit a type 50 Defender pedal box and servo to a Series but you will need some mods. The top of the servo tower may clash with the bonnet and it can easily be trimmed away for clearance. The pedal needs to be slightly shortened and reangled to match the Series but the biggest issue will be the need for a significant wing / mudshield cut, the Defender pedal box sits the master cylinder much further forward. I made all the mods then decided I didn't want the additional cuts.
You cant just bolt the type 50 servo to the Series tower, stud pattern dosnt match. I needed to made an adaptor plate about 15mm thick to place the Series pedal in the correct position and you will need to turn up an adaptor to link the Series pedal to the fork end of the type 50 servo and also slightly reduce the thickness of the existing bush in the Series pedal. Next problem is clearance around the steering box, its going to be tight and I cant say for sure if its going to fit, I did some steering mods at the same time so mine isn't standard, at the very least you will need to rotate the column on the steering box. Its worth noting that the larger servo that Haystee supply is designed for a left hand drive vehicle, it has a cutout on the servo flange presumably to clear the steering box, off course its on the wrong side for a right hand drive vehicle.
I can link you to another website where I compared the two pedal boxes.