1955 Dominator 99 oil tank type/ mounting

Can you post photos of what you currently have?

Oil tank has a vertical tab to be bolted to a tab on the frame.  There should be a small gap between them when offered up.  This is for rubber washers and a shouldered bolt, just like the fuel tank fixing bolts used on all the earlier pre-featherbed frames and at the front of the later slimline.  When the bolt is hard tight, the oil tank bracket is gripped by rubber and not metal.  Your 1955 will be wideline which presumably has a tank strap so you might not recognise what I've written.
Don't know about battery in your case.  Probably similar.
Do you have the support platform?
 

Good morning Alun - If you look on pages 91 to 93 of the Dominator Service Notes Chapter 20 you will see al the different Dominator oil tanks. The battery boxes are in Chapter 19 .  It sounds like yours has the battery strap arrangement rather than a battery box.  The DSNs are on the website under Library - On-Line Library.   Good reading, howard  

Hello again Alun - I think that the image of the battery box that you have depicted is from a bike with the later frame.  When you fit the tool tray you may be able to form a bracket from flat steel bar to pick-up the top bracket on the battery box.  Good luck. Howard    

You can make a bracket by a piece of steel bent round the frame. (don't forget to paint) Even some thing similar to the silencer clamp. And it can even have a thin slice of rubber (inner tube) between the metal and the frame as anti vibration and paint scarring. Saves welding to the frame with all its intendant aggravation. 

is the tank for a slimline frame. yours is a wideline at 1955