You should be able to use your .38/55 expander die for to enlarge a .303 case to accept the 255gr. bullet, you'll be able to use a .375/.303 ctg. for feeding. You may have to taper it's nose slightly, is all, to fit the .303 case neck. This is all I did to several sizes of expander plugs for straight cases.

I've necked .303 brass all the way up to .416 cal., .32, .35, .375 and .416 merely using expander buttons in a single expander die from my .375 Winchester die set.

As a side note, I actually use only .38/55 dies (3 die set - green box) for my .375Win., as .375 Winchester dies weren't available when I bought the Winchester rifle for my wife - back in 77 or 78 - whichever year it was.