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.

Location: Bristol/temporarily Glenelg.

The Longstones are excellent. Have been using them for last eighteen months and have even raced on them.
David

Location: N. Oxfordshire

These 400x17s can be obtaind from Howard Annett at 07860 370307 or 01403 790530 with inertubes and rimtapes

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.

Location: just north of Cambridge