I bought a built 3rd member and the new yoke is about 1/2 inch shorter than stock. What is the proper way to see if there is enough driveshaft in the tranny?
If all you're talking about is 1/2 inch then run it as is. There's plenty more in the slip yoke in your transmission. In my V8 Ranger conversion, the stock driveshaft came up 1 inch too short, I compensated by moving the hanger bearing ( 2 piece shaft in a 89 ext cab Ranger) 1/2 forward and giving the slip yoke on the rear shaft 1/2 less travel. This has worked fine for over two years now, with three motor combinations ( 175hp to around 400 in it now) and two different transmissions ( C-4 and the Toploader 4 speed in it now.