You might be able to do this on the car if you can find a socket big enough to fit ( 46mm) . I would hesitate to say to use the pipe wrench in the car because if you slip you can damage the body or your hands. Neither is very pretty. I converted 46mm to fractional inches...the size of socket you need is 1 13/16" (I think I am correct). Expensive socket for a one time use. Most likely will need a 3/4 inch adaptor to 1/2 inch.Velocedoc,
Thanks for the advice...I'll scrap the channel lock approach. I assume "putting the manifold in a vice" means carb removal. As I have no vice, nore a pipe wrench...this sounds like a job for an alfa mechanic with experience.