The top break frame is not the weak point. You will blow the cylinder apart before the frame gives way. I've never heard of one of these failing due to pressure of a standard 45 ACP load. The 455 Webley was laoded with a heavier bullet than the 45 ACP to just slightly less velocity so the pressures were probably pretty comparable.
As for reliability, the conversion leaves the extractor star rather weak and prone to breakage. The converted revolver will tend to shoot a bit low since the sights were set up for the 455 load. I have seen 45 ACP bullets go through a Webley and have rifling engraved on only one side.
I consider this conversion a butcher's job. the revolver will work better and prodcue better accuracy with the ammo it was intended to use. I've never understood the rational behind the conversion any way. Back when the Webelys were plentiful on the market Dominion 455 Colt ammo was easily available. (455 Colt is the North American civilian version of the 455 Webley cartridge.) Fiocchi markets the 455 Webley today. The 455 was once a fairly popular round in North America. There are a lot of S&W's and Colts around so chambered. If the Webleys had been left stock, the 455 cartrdige would probably be even more avialable than it is.
In my own case, my Webley Mk VI was stock, I had a closet full of 455 Colt factory loads, brass, dies, etc. I was fat, dumb, and happy (Well, no so fat.) until the burglery. I've not been able to find a stock Webley to replace it since.