Is it possible to run an 88 oil pump at 6-start gear speeds?
Sure, all you have to do is put the gears on. I rode a 1961 Dominator for almost 20 years that was all stock except switched to the fast pump gears. Mine had the "S" pump, so you will get less oil than I did, but more than your engine did originally.
It would certainly not hurt to enlarge the oil passages the way Norton did when they went to the large pump and fast gears, but not doing so did not hurt my 61' Domi over tens of thousands of miles of road use. I did read somewhere that putting the fast gears on an early Domi with the large S pump could froth up the oil, but I never noticed this problem.
On my current daily rider, a 62' Domi, I have the S pump and fast gears installed also, but it also has had oil passageways enlarged towards the 66' and later specification, but not quite there. It has the 62' and later oil tank with the froth tower too, which can not hurt and gives a place to connect the crankcase breather to.
One could look at the factories oiling system modifications for the Norton Dominator as layers applied over the years. The first level is the small pump and slow gears, the second level was the big pump with slow gears, the third level was the big pump with fast gears and enlarged oil passageways. If you want to count the 850 Commandos addition of a spin-on cartridge filter then add another layer.
On the top end there were two steps, the original low-pressure scrolled rocker spindles fed by the return line to the oil tank, and the 1966-on plain spindles fed by the pressure side of the oil pump. Both top-end oiling systems have been used with success on any combination of oil pump and drive gears, but with the early scrolled spindles you have to check you are getting enough back-pressure in the return line to force oil to them.
Whenever you swap or mix parts from different years it is your responsibility to do the checking on the final installation to make sure everything is getting lubed. Just because someone else has got a parts combination working, does not mean that the parts you bolt on will do the same thing. Something as simple as the size of the oil return hole in the oil tank, or the amount of wear in your oil pump could give completely different results, and with many of our parts being half a century old no one can guarantee what your parts will do for you.
Also you should get hold of Hamish Cooper, he is a journalist in Australia/N.Z. who has a road racer with a Norton 88 engine in it along with many spares. He could put you in touch with the Norton Model 88 network "down under". If you google him you will be able to find his website and contact him.
Good luck
