Les,
I have used these guys for several years. Good support and great prices. Usually the old gas spring has the sizing information you need printed on spring. If not, you can measure for the size and take your best guess at the spring pressure you want. I use a bathroom scale and push down on the spring to determine the spring rate. If testing a tired one, I just add to that number what I feel would be appropriate. Now you can't, at least I couldn't, compress the struts for the engine hatch cover without mechanical advantage. I used a hydraulic jack to compress that one.
Hope this link helps:
http://www.spdhardware.com/
Russ