In carbon arrows -
To increase the spine,
1) Reduce total arrow length
2) Increase knock end weight
3) Reduce point end weight
To decrease the spine,
1) Leave the arrow as long as possible
2) Reduce knock end weight
3) Increase point end weight.
Weight tubes/nylon rope inserts will not effect the overall spine, though they will add mass (weight) to the shaft.
For bow weights from 40-50#, start with a full length .500spine arrow. If closer to 40#'s, leave the arrow full length and slowly add point weight while bareshafting. If closer to 50#'s, cut the shaft to the desired length and add point weight to bring it in. If you want to use a short arrow with a 40#'s, you will need to either go to a .600spine carbon and start adding weights to both the knock and the tip, or severely overweight the tip of a .500spine arrow (+25%FOC).
GoldTip, and PDP make weight systems that work well in most .245-.247" diameter arrows (GT, Vapor, Beman non MFX, CX, etc...) and have systems to weight both the tip from the back of the insert, or the knock from a modified bushing.
Good luck!