C20 driveshaft shortening

[Jackrabbit760]

Hey there fellas I got a few questions I'm hoping you guys can answer for me. I have swapped out the old eaton for a dana 60. Seems that i will have to shorten the driveshaft some. I pulled the trigger rather quickly and cut the yoke of the rear driveshaft I hope that wasn't a mistake. It's a 2 piece so I'm wondering what shaft would actually need the shortening. Also I have the truck on jack stands under the chassis a couple inches forward from the axle and have my jack under the axle will this give me a false reading when determining how much to shorten on the driveshaft ? Should the truck be sitting on all 4 wheels instead? The truck is a 66 c20 with coil rear end and trailing arm suspension 4 on the floor granny. Thank you much and look forward to seeing some replies.