The 1979 K frame was a clean sheet design as compared with the 1969-1976 cb750K frame and the 77-78 updated 750K frame. One of the things that I did not like about the frame is it now included a drop away down tube to make removing the engine easier. You cannot put a SOHC engine in a DOHC frame, it is way too much to be worth it.
There are three variants of the 1979-1982 DOHC cb750 frame:
the cb750K - standard frame. good all arounder but does nothing particularly well. The frame itself is not crusier but the seat does have a slight drop to it. The K bike is a physcially bigger bike than the SOHC that came before it. It is an upright standard riding position stock but it can be modified to be more cafe racer.
The cb750F - Essentially the same design as the K frame but it uses a thicker wall tubing and is much stronger. Also has the monuting tabs for the rear disk brake setup the F model supersports used. This is probably the best DOHC frame. This was honda's factory cafe racer/race replica to combat the kz750/kz1000 and suzi GS.
The cb750c - This is the crusier frame. It has dropped seat rails and a different rake. Will take a lot of work to even make look like a cafe racer.
With the DOHC bikes honda also made 900cc, 1000cc, and 1100cc variants. The cb1100F is considered one of the best of the last of the air cooled supersport hondas. The 900F has a lot of nice little tricks on it like factory oil cooler and larger petcock. The 900 and 1000 Customs have 10 speed transmission having a conventional 5 speed gear box coupled to a underdrive/overdrive (high/low) unit and a shaft drive. As far as I know there are no 900 or 1100 K bikes but the F engines fit with a little work.
The 1977-78 K bike frame uses the same headstock as the previous K bikes but they changed the back protion of the frame to accomidate a bigger seat and tank. This makes the bike look bigger but stripped down the frame is actually smaller than a 79K. I personally don't think very highly of the 77-78 K bikes because they are heavier than previous SOHCs and although they make more power than previous SOHCs they DOHC blows it away.
for SOHC stuff check out www.sohc4.us
for DOHC stuff check out www.cb1100f.net
