Of course as soon as I need to get my truck smogged (by end of April) I find a crack in my stock exhaust manifold on the the driver side. Also, my cat is rattling.

Other than the visual inspection, will a cracked manifold cause me to fail? Also, will the cat make me fail if it just rattling and not melted/coated with oil?

Can the cast manifold be welded until I can get it replaced? What about JB Weld to seal it up, will that work? All until I can get it replaced.

The truck runs great, just sounds a bit off with the leak. Its a 97 Dodge 318 with 120,000 miles on it.

The crack can most likely be welded if it is not to big. Hell, I would do it even if it WAS big. Some folks do this on yotas and get by for another 10K or so.

AS for JB weld...ahhhhhh NO. I have never found anything that JB weld was the best solution for. Still looking:D

Best way I've found to fix an exhaust manifold is to braze it. Tried to mig one once and as soon as i tightened the bolts it cracked the weld.

Gas weld it or braze it. As to the question of will it fail the smog I would say yes I bet it would due to the leak but not for the cat.