Longstones tyres are excellent.
If your Opal has a painted "Ruby " style rad cowl then 400.17s are the prefered and correct size.
However they also made some Opals in 1936 with the earlier chrome radiator cowl on the old high frame chassis and these cars should have 350×19 wheels and tyres.
I had my last Ruby for 38 years and always used 4.50-17, but finally I bought two 4.00-17 for the front. It was transformed: the steering was comfortably light compared with using the larger section tyres. If you use 4.50 on the back you raise the effective gearing slightly, which is fine, but 4.00 on the front are definitely better.