If you've got an 8 3/4" rear and a proper E body TF727 then you won't have to worry about measurements Chuck. Most any Mopar salvage yard will have a core that you can put new UJ's in to.
Driveshafts are normally measured from either the output shaft from the Trans or the Yoke to the differential yoke. "Knuckles" are those guys that drive pherds or shivs,,,overseas they are called Pompous Bloody Wankers. you need approx. 1/2 to 3/4 inch 'travel'
You should try and use at least 7290 U-Joints,, this is what the factory did on 440 engines,,there is only 1 brand I recommend,,,SPICER!!! Don't use overseas Joints,,I've never seen any that held up,,at all!!!! Make sure they are installed correctly,,you can put the C-clips in the wrong way, it will work for a while then POP!!
Cool Beans Chuck! Insert it into the rear of the transmission output shaft,,then slide it to the rear differential yoke,,you should have 1/2 to 3/4 inch sliding clearance. This should be done with the weight of the car on all 4 wheels,,this will allow for running clearance as the shaft will move in/out as the suspension is "working".
Find a used shaft then take it to a driveshaft shop and they can cut it and reweld! Use your tape measurement and add 1/2 inch to that. That's the length you need,,,locally it's 55 bucks to cut shorten and reweld/balance. Saves a lot of money that way!
Chuck,,,try my friend Steve at Steve's Musclecars and parts Alma Ark. 72921 his number is 479-632-0011 he may have exactly what you need,, a used shaft for a lot cheaper than anew one or a cut/welded one. Good Luck