You'd be far better off using a Saginaw "525" box rather than a Vega. A dozen guys are gonna come back saying that the Vega works just fine. But that Vega box WAS engineered for a wimpy, light Vega, and a '40 is a heftier vehicle than a Vega. The "525" is shaped very similar to a Vega, yet is only very slightly larger, and most-definitely rugged enough to handle a vehicle of a 1940's weight. The "525" came in GM mid-size vehicles such as Chevelles and Cutlasses. They're great boxes and can be had in several ratios. Also, the '40 Pitman arms fit the '60s, '70s & '80s "525" spline. And, they can easily be bolted into a '40. Borgeson and many others have all of the adaptive and support pieces you could ask for. DD
I used a short input shaft Borgeson 525 box from Pete & Jake's in my 40 coupe and it works great. I used the bolt in frame adapter #5002 from Pete & Jake's to mount it. Also used the P&J 525 pitman arm. Costs no more than a Vega and is much safer
If you insist on using a Vega box, it would be a good idea to stay away from the aftermarket boxes. There have been many reports about them failing prematurely.
If you insist on using a Vega box, it would be a good idea to stay away from the aftermarket boxes. There have been many reports about them failing prematurely.