The standard Spitfire / Herald type front uprights have trunions and are widely adapted for small racecars. They were even used in F1 during the early sixties. That was probably plain scary, but anyway...
There are pictures here on this site. If you can't find them, I could repost them here. The last time this was discussed was in EarkeyMotorsports thread about building a little formula car.
On those uprights the trunion was on the bottom and it is removed just leaving the threaded portion. That part is either turned down or cut off and a stub welded in it's place. A normal spherical bearing is then used in a cup on the end of the wishbone. This prevents putting any threads from a rod end under load.
So far as your welding table preventing frame distortion, that will not work. It is good for the parts to be well located, but be carefull with your welding and keep track of where you put heat into the frame. Alternate sides etc. When the metal is very hot from welding, it expands but the red hot part will just compress because it's soft. As the metal hardens it will contract but it cannot change shape anymore so the shrinkage will cause it to pull tension. Clamping it down won't change that, at least until the stress actually causes the metal to yield or bend.
Glad we have some more Formula Ford guys here...