Installing front cab support on 64 pickup

[bluesbrother]

I'm replacing the rusted out cab supports, floor, and rockers on my '64 pickup. When test-fitting the front cab support, I found that there is about a 1/2" gap between the "ears" and the floor support. Also, it doesn't mount at exactly 90 degrees. I braced the door jamb before taking it apart, and I remounted the door to confirm that it still fits OK, so I don't think the cab has shifted while I've been working on it. However, who knows if it was really square before I started?

If anyone else has encountered this problem, I'd like to hear what you did about it. How can I weld the cab support to the floor support? Should I fill the space with a piece of thick steel, or do you have any other suggestions?