Whats the max dia. disc that will go inside radial mount Willwoods on RMD alloy hubs with 15" wheels ?
Also whats the general opinion on vented or non vented ?

I havent got quite the same set up but have cortina uprights with Wilwood calipers and 15 inch rims my disc is 265 maybe that helps you ?

Should be able to get more than 265mm. I have 275mm on 13" rims but it was a struggle.

The key is the internal diameter of the wheel, where the tyre fitting well is and the lateral location of the caliper. Ask for a section of the wheel from the manufacturers but they are very coy about handing this info out.

Agents, in my experience, are worse than useless and simply do not know their product. I am resorting to taking a mock up and seeing if it will fit to try and get a few extra mm!!

Cheers