Hello Geoff,
I can't say how radial tyres perform on a 2-litre as the 403 that you so kindly sold me in 2013 is still being restored (I am entering the contest for the longest laziest restoration in the world) but on my 409 the adoption of radials changed the car completely. Admittedly the crossply Avons were quieter but the road behaviour of the car was erratic, and the high-speed stability, tramlining and performance on the wet all improved immensely with the 'new' 185/16 radials. I adopted the Vredestein Classic Sprint tyre:
Bristol 409: Vredstein tires
They are still on the car and 10.000 miles later they still as good as the first day I fitted them. I shall try the Avon Turbospeeds again on the 403, mine are still practically new, but I'm sure that I shall switch to radials also on it; I'd buy Vredestein again as Michelins are surely very good but cost twice as much.
All the best
Stefano