No HD's, except for the V-Rod, have CRANKCASE oil capacitites. They are a Dry-Sump system that use remote oil tanks.
If you don't know this, or the capacties, or where the different fluids go........you don't need to be doing your own work. If you MUST do this yourself, buy a FACTORY SERVICE MANUAL!!!!!
The oil TANK capacity will vary depending on the model and whether or not it still has the stock oil tank, generally 2-1/2 to 3 quarts. The oil filter is also different, some used Drop-In style cartridges in the oil tank, and some used remote Spin-On style filters.
Off the top of my head, I can't remember the trans capacity, I always have to refresh my memory with the manual, I think it has been over a year since I last serviced a Big Twin 4 speed trans.
You MUST use GEAR OIL, reommended is 80/90wt.
YOU NEED TO BUY A MANUAL OR BETTER YET, TAKE IT TO A SHOP..........BECAUSE......
with it being a 1975, and you did not specify model, or even if its a stock frame, etc, AND that is has an open primary, which was NOT stock, this bike has obviously been modified and is not stock, you obviously don't know what you are doing and possibly don't even know what you have and how much is stock.........take it to a shop and let tem service it and have them give you all the details as to what you have and what needs to be done in the future, the capacities, specs, etc.