I'm no rocket scientist and I reckon that replacing seats is a fairly easy and satisfying job. You just need to be well organised before you start chopping and welding the rails under the seat.
I've always kept the original rails (detatch them from the seat )and then made up a square steel frame from say 50 x 6 mm steel .Then drill holes in the square frame to suit the new seats and fixing bolts. Then weld the rails to the square frame .
Thats it in a nut shell. Just ensure that everything is dead square , parrallel , and straight otherwise if the rails are in any way twisted or out of line the rails will not slide. Keep the seats as close as possible to the center tunnel . And whatever you do keep your welding away from anything that can burn !
Once your seats are in place you can easily shift the (fixed) position by redrilling the holes that fix the seat to the square plate.
In a worst case scenerio , if you totally hash it up , all you have to do is get a new pair of runners from the wreckers and bolt them back on to your old seats !!
Seat mods also require a blue mod plate (in Qld )so you may need to talk to a mechanical engineer who is qualified to do the mod plate. Do this before you start