SAAMI pressure for the .45 ACP is 21,000 psi, well above the 14,000-15,000 range for other big bore revolver cartridges of that era like .44 Special and .45 Colt. I'm sure millions of rounds of .45 ACP ammo were fired through Model 1917 S&W's and Colt's during WW I and I've not heard of any problems. Having said that, I only fire low pressure handloads through my guns from that era in deference to their age and lack of heat treatment. I've got plenty of modern N-frames if I want to fire heavy loads. Interestingly, SAAMI pressure for the .45 Auto Rim is only 15,000 psi.

I would definitely not convert your gun from .455 Webley to .45 ACP. I shoot a British-proofed S&W 2nd Model HE that shipped in 1915 and has the officers name engraved on it and would never alter it.

Both Hornady and Fiocchi make ammo, but it's about 80 cents/round. As far as I know only Hornady makes brass and it's only sold through Grafs for $30/100. I bought 200 5 years ago and I've not had a single case split with 10 reloadings. I use the Remington flat point 250 grain lead bullets; they're .456 diameter, which is perfect for this cartridge. I've heard that the Fiocchi brass is very thin and splits easily, but I've never bought their .455 ammo, so I don't know that from experience.

My gun is a pleasure to use and shoots to point of aim at 50 feet. I can shoot just as well at bullseye targets as with a modern .45 revolver (but not as well as my Model 52!). I get a certain satisfaction from shooting guns that have some historical significance as opposed to modern guns.

Have fun!