have you tried posting the same question on hondachopper.com? there is at least one guy there who has tried the later gw front end swap and maybe can help you.
I thought 78 was the mag wheel on a GW. When I look up fork seals and headstock bearings I always see it as 75-77 (76-77 in some cases) sO I always assumed it is different. For doing the 75 swap I have, it takes one cb750 temkin bearing ( I think the lower) and one GL1000 temkin bearing (I think the upper, but I would test fit to make sure). If the bearings are listed as the same for the 78 - I'd try this approach first, you might find the solution is that easy.
I had a friend who was into kawi z-1s, and also worked in a bicycle shop. One night we raided every ball bearing in the joint and started expirementing. The end result: with two different oddball bearing sizes and two flat washers we were able to adapt a ducati 916 front end to his 1975 kz900 streetfighter project (and yes it was safe to ride). This only works on bikes where the original bearings are not temkin bearings (like your GL1000 and cb750 front ends). I know that in racing some of the fzr400 guys use bearcans to shim their headsotck bearings when the headstock goes out of round so that might be a possible source for shims.
Most of the guys on this board seem to be more into the stock stuff, so custom stuff doesn't get that much of a response. The best way to find out if a GW front will fit in your cb750 is to get your arse in the garage and start messing with it.
