ok first question is this an F bike or a K bike? The reason is that K and F bikes have different exhaust mounting flanges - K bikes use the clamp tighening around the pipe method while F ones use a lip and bolt to the head.
From the looks of it I's say you have an F exhaust header. There are no internal inserts in a cb750 exhaust. What your pictures show is a really backyard repair of a broken header pipe. The cb750F headers are supposed to have a lip on the end of the pipe to which two shell pieces attach and collar slips over. Usually if the bike is running lean the pipes will overheat and these lips will break off. I have a cb550 and 750F that it has happened to.
As far as fixing it, welding would probably be the best and strongest option (basicallt welding a new lip and throwing that insert away is ideal). I don't like the backyard fix because it restricts exhaust flow (punches up the lower end but you lose top end and overall power). As a For now fix I would consider sliding that insert in and riviting it in place with pop rivits (make sure the colar fits over the rivist and doesn't get in the way). The header is pretty much junk anyway so any expirementation to get it to work isn't bad. Ebay for a good used header is not a bad idea, I scored one last year off ebay for $35 flanges to silencer and after a coat of hightemp black it looks better and badder than ever (espically with some heat wrap covering the dents).
In a pinch MAC makes a cheap Super Sport exhaust but requires you to switch to the shorter oil filter cover to clear the pipe.
oh and if it is a supersport (F bike) you don't want to ride too long with an exhaust leak on those F heads - you'll burn a valve before you know it. According to my friends (ex honda service techs when the bike was new) the 77-78 F head only has a service life of 20-30K before it needs to be rebuilt with new spings, honda put bigger valves in the F head for 77 and the springs are not adiquate for the 50-60K mile service life of previous cb750s.
Edited by - geeto67 on Aug 09 2006 4:11:15 PM