1988 GMC TH400 slip yoke to 1973 driveline?

[raceman6135]

Conversion universal joints are available.

Measure what you need, and check this chart to find out what 'series' each cross of the u-joint has:
http://www.driveshaftspecialist.com/...D%20Guide.html

Then, say for example you need 1310 series on one side and a 1350 series on the other side, scan down this page until you find it:
http://www.northerndrivetrain.com/ca...ionJoints.html

You will need a Spicer 5-460X. Take that part number to your favourite auto parts store and buy it, or their equivalent.