3/4"-36 spline. If you're wondering about which universal to get, this should be helpful. Quoted from Mwilliams: THE ONES IVE USED HAVE A SET SCREW THAT TIGHTENS DOWN ON THE FLAT SPOT OF THE SPLINED STUB. UNLESS YOU FIND ONE THAT USE'S A PINCH BOLT LIKE THE FACTORY ONE , THEY WILL SLID OFF.. SO STAY AWAY FROM THOSE..

SPEEDWAYS PART NUMBER FOR THE 3/4 SMOOTH BORE ON ONE END AND 3/4 36 SPLINE ON THE OTHER IS 910-850 ... THEN YOU CAN JUST CUT THE RAG JOINT PART OFF THE SHAFT AND SLIDE THE JOINT ON THE SHAFT AND WELD THAT END, AND THE OTHER END IS A PINCH BOLT STYLE.. ALWAYS WORKED SLICK FOR ME..

Read more:http://we-crash.proboards.com/index.cgi?board=ford&action=display&thread=13369#ixzz1n8dDHiRz

Depends on your setup. I have used the stock Astro squeeze type u-joints for years and never had one come loose and they are not even the right size. They are only 11/16 not 3/4 like the ford boxes but they work anyway, whats a 16th on a derby car anyways

http://www.speedwaymotors.com/Safety-Racing-3-4-Inch-Forged-U-Joints-36-Spline-to-Round,6632.html

They sell colars that lock onto your shaft with a set screw. I usually just tack weld them to the heim or a spot in front and behind the heim.

Where's Lenny?

I usually just weld them together.

I just got the u joint from speedway and it fits really nice and it
Was the cheapest price I could find. So if your building a columb they might be something to look into.