Well I have not modified too many drivelines, but I did work at a driveline shop for quite a while and if you really did want to change it (which i recommend not) you take your drive shaft to a specialty shop and what they will do is cut a length of tubing and weld your mateing yokes from the old drive shaft to the new, also they way you measure a driveline is from the center of the u-joint bearing cap and from weld to weld for good measure. I also recommend measuring from the tail shaft of the trans to the mateing yoke on the differential and based on your ride height you will get the amount of end play you want your slip yoke to travel. for a stock chevy c-10 i would go with anywhere from 1 1/4 to 1 3/4 inch of travel but only if your slip yoke splines are between 4 to 5 inches if is smaller step it down to about 1 inch or less. these measurements are to be taken prior to drive shaft welds are completed.
