So my Dad passed about 3 years ago leaving me a 1980 Honda CB 750K.  These were awesome for production and performance bikes.  So I chopped it and dropped it!  I turned it into a bobber, I am stoked the way it came out.  I am using to run back and fourth to work and just having fun ***  I want to drop the RPM's when I am on the highway.  I live in Arizona so there are some great desert rides but since it is a hardtail now I am going to rattle my teeth out of my head at 70-75 across the desert.  It is the stock 45 tooth, any recomendations?  

 There are many CB 750 forums out there. I recomend that you go to 1 tooth smaller Rr sprocket and 1 bigger on the Ft. Rule of thumb = 1 tooth Rr about 3 miles per hour, 1 Ft tooth is about 6 Miles per hour. Hope this helps you.
Then you need some extra chain links. If the chain is worn then you will need new chain. If it's too worn then the sprokets will prematurilly ware out so fast...........get a new chain EH !!!!

Ride on !!!!!!!!!!!!!!  

to lower the engine speed at say 75 mph, you need to change either the rear sprocket to one with LESS teeth
( I would try it in incriments of 3-4 teeth at a time)
or you need to put a LARGER sprocket on the front, at 1-2 teeth at a time, remember that the front is easier to change, but because of space issues, you have a limit on how far you can go in front. If it were me, You say that it's a 45 tooth in rear? then unless you have given the engine at least %30 more power don't go more than 3-5 teeth in back, or 1-2 in front, as honda engineers are very good at getting the gearing on their machines just right for the power to weight ratio. And unless you have really upped the power, you will not like the way it responds with a 5-7 tooth change, not to mention a lot less fuel mileage, however if you have given it a lot more power than a 4-5 tooth drop will give you better econamy, and higher top end, but slower 1/4 mile times.