There is quite a lot of random information about tank paint templates and much of it is incorrect so don't take the first piece of advice you get, keep asking until you have a level consensus.
For example, the template on the Danish site for 1937 is incorrect.
http://www.arielklubben.dk/tank_templates.html Also, the pattern for a large 500 tank differs from the smaller 350 tank.
For the 1937 season when in the saddle looking down at the top of the tank, the design is vaguely similar to this pattern. (This example is an Indian reproduction so don't take it too seriously)
http://2.wlimg.com/product_images/bc-fu ... 886959.jpg
The 500 tank should have additional paint at the very front where the paint on the top runs down the front. Here is a photo of my 38 tank which has the same.
- tank.jpg (41.23 KiB) Viewed 3829 times
Also, the panel in which the instruments are mounted should be painted (not chrome)
Regarding exhausts, there are some discussions on this forum which you can search for especially about the Armours items and how well they fit.
https://www.armoursltd.co.uk
Some heads were single port but it was the right hand side which had the pipe. I assume that someone lost the right pipe so they just blocked up the hole.
The panel here will no doubt make some further observations on some of the parts on your Ariel such as the later forks, unpainted rims, post war headlamp, missing rocker covers etc...
For further reference here are some photos of my old 37 350 which has the smaller tank and the top of the tank has the 1938 pattern so don't copy anything from this tank.
http://www.barkshire.co.uk/bikes/images/ArielNH_1937
