Coil springs or coil overs?
Either one will work for what you are doing and if tuned correctly both will work just fine. Lets address cost first. You already have the coil springs and you would have to buy the coil overs. With coil overs I have never been satisfied with the first set I purchased and usually had to go through about three pair before I found the ones that worked the best. The same holds true with coil springs but you can get them from the junk yard. The problem with coil springs in a drag car is if you don't secure them in the perches they can jump out on a good hooked up run when the axle winds up. In a completely stock arrangement the length of the shock is supposed to keep the rear axle from extending to the point where the spring jumps out and under normal circumstances it works this way. But I have seen shocks break and the rubber bushings give to the point where the axle winds up and the springs jump out. You sound like a good fabricator and you should be able to make a way to hold the springs in their perches. Look at the front suspension on a late sixties through early eighties F-150 Ford pick up trucks. They had a real good way of securing the springs and you should be able to duplicate something like this to hold yours in place. You can also run a 3/8 inch cable inside the spring from the upper perch to the lower perch. Get someone to crimp a heavy duty eye (loop) in each end. Secure it to the top perch with a cross bolt and then use a cross bolt setup underneath and in the center of the bottom perch and make this one easy to remove. Measure the cables to leave enough slack in the length so that when the suspension is extended the coils are a little bit loose in the perches. You will have to do this with a floor jack under the rear and jack stands under the frame. Another thing: you don't have the adjustability that you get with a set of coil overs. At the track you can set pre-load, right side bite and ride height from one side to the other just by turning the collar. With coil springs you have to play around with the length and you will have a lot more trial and error. Coil spring are heat treated so don't use a torch to cut them. Use a die grinder with a cutoff wheel. One thing good about coil springs is that they do ride a lot better then coil overs. So it is a give and take scenario and it will take more work to use the coil springs and more money to use the coil overs.
Sorry but I am not much help on the re-spline. Here in Jacksonville we had an old Guru that used to cut the housings and re-spline the axles for us. He was a retired D.O.D. machinist and he was awesome. But he passed away a few years back (greatly missed) and I haven't done what you are doing in about five years. You could how ever take your axles to all of the machine shops that are available in your area and explain what you need. It is not that hard to do you just need the right equipment and just about any machine shop is capable of doing it.