Sunshine, the 645 cameras also have the standard matte focusing screen. They do however give you focus confirmation on the auto-focus bodies. I have a 67ii with a split image screen (as mentioned they are easily swapped on that model). I do find my 67II to be brighter than my 645n, someone else may speak to brightness of the 645nII. You could probably solve your focusing issue trading up to a 67II .
Here's a fun idea: If you think its hard to focus your 67, try your hand at large format, that will turn you into a pro
Since you said your not worried about the smaller negative, what it really comes down to is how much you want to spend solving the problem or switching systems. The 645 lenses will cost more, especially if you go for the newer FA lenses. Switching systems would allow you to shoot more frames per roll so it may be good for you in that respect. I would hang onto your 67 system if you try the 645, at least long enough to decide if the 645 is for you. Start out with a 645 and the 75mm before jumping all the way into a system switch.