Exhaust Info Needed

[MikeIIDC@domain.elided]

Help!  I have taken my '71 Scout 800 in for new exhaust system.  The
guys recommended I not do anytiihng because the engine was "knocking" a
little and needed work first.  I have the wrong carb on and it is very
rough, but I don't really have any fear of the engine going bad-I have a
spare 345 to drop in in its place if I need to.
My question is this:  Would there be any prob with keeping the same
exhaust for the 345 as the 304?  I was thinking that I would go ahead
and get it done, rebuild the carb, and then see if I couldn't figure out
more of the engine noise after I could here something.  Are the
diameters of the exhaust systems different from one to the other so that
this can't be done?  I need an answer pretty quickly-before 5:30 tonight
if possible (Pacific Time), so if anyone is on, please give me some
feedback ASAP!

Thanks!

Michael