A 38mm actually will fit in the stock boot without too much trouble and you,ll likely have to go richer
due to the lesser signal of the bigger carb.

I've gone down that road, yes it'll fit without too much trouble, but if the intake boot is old it'll crack or tear, and if you leave the carb on long enough it'll eventually cause the boot to crack.
The zilla boot was designed for that size carb, so it'll fit without any problems and it won't stretch the boot out and cause it to crack prematurely.
Not only that, but the intake boot is larger and flows better.