Hate to beat on this one, but when I swapped a 8 1/4 for the 7 1/4 in my M-body the factory dimension for the 8 1/4 was off by almost a 1/2"!
This was because MY car was slightly different. And it was a costly mistake because I had to pay to have the driveshaft cut/balanced/etc twice. $100 x 2
And I felt equally stupid because it took all of 10 minutes to measure it and get it right- and because I had the original shortened too much, I had to find another shaft for them to cut- which was another $30.00.
So- Learn from My Mistake- measure the length you need and don't depend on dimensions you get from the internet or other 'sources'. Your car- like you- is unique.
When you measure, be sure to put all 4 wheels on jackstands with the suspension in tension- like it is on the ground- or the measurement will be off. Then it is as described above- measure center of u-joint eye to center of u-joint eye and pull the yoke out about 1/2-3/4". This allows for in/out movement when the suspension flexes.
I did it the super simple way-
Took a piece of string, tied it on the yokes, marked it, then took it out and measured between the marks, then added something like a 1/2" for each side (total 1") (sorry don't remember exactly- this was 5-6 years ago) for each side to meet the middle of the joint.
Good Luck,
Hope this help,
CC.
