Yes, you can have them shortened, no sweat. I have actually done it myself (when I worked in a machine shop

).
Measure from U-Joint flange center of rear axle to center of U-Joint mounting in the slip yoke of your tranny. Make sure your slip yoke is all the way forward in trans. and deduct 1 1/4"- 1 1/2" ( minimum is one inch!). That is your final dimension.
An El Camino (preferably '67-'72) is a good donor car.......it has a long driveshaft (not sure about 61", but it probably is!)

.