Finally finishing up t-5 conversion , I have 3/4 of an inch of driveshaft freeplay with the car on the ground. Is this enough or should I have it shortened alittle more just in case?
There are 4 measurements you need.
Find a car lift, the kind you drive up on. Block the front tires. Mark the location of the end of the trans on the yoke with a felt pen. Disconnect the drive shaft from the rear end and push it all the way in the trans. Mark this location on the yoke with a felt pen. reconnect the drive shaft and pull the rear of the body down as far as you can, (To the bump stops if possible) mark the location on the yoke. Release the body and jack the body up letting the rear wheels hang free. Mark this location on the yoke.
If any of these marks are to close to the end of the yoke or to close to the U-Joint end of the yoke, then you must shorten or lengthen the drive shaft, if the marks fall somewhere in the middle, Let the car down and do a burnout when you leave.
You need .750 to 1.000 inches of freeplay with the cars weight on the suspension. You can put jack stands under the rear axle to give you room to get under the car to measure.
Too little free play and you risk driving the slip yoke shoulder into the transmission case. Too much free play can cause vibration and damage to the bearings in the transmission.
I took some pictures and have some information from when I did my T5 conversion on my website: Drive shaft