CB750 Tank on a CB650 frame and other help

Hi- I have not done anything for a year with my project due to some issues anyway I ave a 750 tank that i wanted to use but its a bit long for the frame and the bolt hole is not lining up in the frame and the tank, does anyone have any mod ideas for this? also I want to put the battery under the new Vincent seat i got, any ideas on how to secure it to the frame. I also have a CB400/4 engine, smaller than I wanted but there you go, will this be ok in the 650 frame, I have some carbs off a CB500 twin would they fit the 400 engine?

The frame is a 1980 on a V reg

What year and shape is the 750 tank? Post '76 they went for the long, square'ish shape with the flip-up filler, the older pre-'76 tanks are more oval and probably shorter.

Having said that modifying tanks is not an easy job and certainly not for the inexperienced. Can you not find a 650 tank? That would solve your fitment issues.

Siting the battery under the seat hump shouldn't be a problem (although without seeing it it's hard to tell), try welding a flat plate or shallow tray beneath and between the rear seat support rails to sit the battery on, that should drop it enough so it doesn't foul the hump, then you should be able to fabricate some sort of clamp to secure it. Alternatively you could go for a gel battery that can sit on it's side to give you more clearance.

400 into 650 shouldn't be an issue. You may have to fabricate some engine plates if the mounting lugs don't line up, just ensure the chain run is true or you'll eat chain and sprocket sets! As for the carbs, check the manifold fittings, if they are the correct size and spacing they should fit, and you can always jet down to allow for the difference in engine size.