440 or 750 Twin engine in GPZ550 frame

From what I'd like to set up in a 440 cafe racer it would almost seem easier and less expensive to find a GPZ550 with a blown engine and put a 440 engine, or possibly a 750 Twin engine in it's place? I absolutely do not want a 4 cylinder engine, period. Any thoughts? Has anything similar been done?

Seems like it would be a lot easier to use the frame made for the engine. The 750 twin is a pretty tall engine. It may not fit in the 550 frame without substantial modification.

For me, the 440 "A" and "D" models would entail replacing the rear wheel for an 18 inch wheel with a disc brake. The frame is unsuitable for a flat seat. The gas tank is too small.
The 440 "C" model would be ideal in stock form but they seem extremely rare in the US. An alternative, for me, might be going with the "B" model but I'd want to ditch both wheels for mags and have a front disc.
The 550GPZ has beefier front forks, 18 inch rear wheel with a disc brake, a bigger gas tank and the seat and handlebars I want. It has everything I want EXCEPT a 2 cylinder engine. 
(Since I was broaching the idea of switching to the 440 engine in the 550 frame I just, as an aside, mentioned the 750 Twin engine in case anyone here had done such a thing it might provide additional ideas. We can just skip that whole idea).
When I compare what I'd like to switch over TO a 440 I just started wondering if it made more sense to get the chassis I want and just replace the engine?
Gas mileage is important and the 440 would be much more appropriate today. My very first motorcycle was a brand new 1980 KZ440A1 which I put over 33,000 miles in a little less than 3 years. I drove it from FL to WY and NM to FL without any problems and a few other 800 mile trips. I kept track of every drop of gasoline that went into that bike and most tank loads were 60mpg+. 

I do not remember anybody here putting a twin into a four cylinder frame, so you will likely be blazing the trail.

My 750 twin never got that sort of fuel mileage, so if that is a high priority, the 400/440 looks like a winner.

I look forward to your build thread.

No offense but that is an ass backwards way of doing things. Much easier to swap suspension, gas tank, etc, than swap a twin engine into a gpz550. If you have the skills to fabricate custom engine mounts and probably chop the crap out of the gpz frame a simple suspension swap should be no problem. If you are worried about cost use find a gpz with a blown engine and swap all the running gear

I can not remember ever reading about anyone putting a twin into a 4 cylinder frame. You'd think someone would have felt "forced" to do this by finances or some contrarian would have done it just to prove it could be done. I've researched this online, including here, and all I've been able to find info on are folks doing the opposite of what I'm interested in, namely, it seems like everyone else wants to stuff a bigger engine in a smaller frame. I've not invested in this idea beyond internet research and asking here. Of course, a middle road would be to buy a decent 550 GPZ with a nice body and blown engine and work on switching over the tank, forks, wheels and seat etc. to a KZ440B.

I already posted a response coming to that same conclusion. My thinking was that a 550 engine being slightly bigger might make the substitution of a slightly smaller engine much easier. I appreciate the feedback!

Just out of interest, what is the exhaust port spacing on the twin compared to the down tube spacing of the four cylinder frame ?

I would think comparing the head from a 550 and a 440 might tell me if that end would work? Of course, the downtubes are not an equal distance apart (they're splayed) so I'd also need to get an exact measurement of the 2 engines exhaust port height as it sits in the frame to figure out if they'd clear those downtubes. I probably would have a much better idea about any of this if I went to a motorcycle junkyard.

wrote:

I already posted a response coming to that same conclusion. My thinking was that a 550 engine being slightly bigger might make the substitution of a slightly smaller engine much easier. I appreciate the feedback!

The 550 block, head and crank are a bit wider, but I bet the transmissions are close to the same width. The 400/440 may be a bit taller overall since it has a rocker arm style valve actuation, whereas in the four cylinder bikes the cams press directly on the valve stem via bucket and shim.

NOBODY sees the elephant in the room.
Chain and sprocket offset from the center line of the bike is smaller on the KZ400/440s compaired to the 4 cylinder bikes.
I have  put KZ650 forks and tripples  along with a KZ550 swingarm on a KZ400.
This gave me Twin disc up front  and 36 mm forks. On the front a KZ650 hub with a KZ400 rim. For the rear I used a KZ550 swingarm[narrowed] with a KZ400 hub in a KZ650 rim.
This is with spoke wheels. Since you want mags you will need to do what I did on one of my current projects.
I machined the hub from KZ750LTD down to fit a KZ440LTD and used a different width space to square itall up.

Thank you for the thoughtful reply! You folks are steering me in all the right directions.
From the For Whatever It's Worth Dept ... your user name and icon are hilarious!