carpet will hold moisture. moisture makes things rust.
unless you live in an arid climate, or have all new seals, i would avoid the carpet and go with FLFJ40's suggestion to put in a bedliner.
when i bought my 40, the only thing that saved the tub from rusting under the carpet the PO had put in, was the oily goo that had been trapped underneath. to this day i'm not sure what it was but, it sure was a pia to remove.
If I had a cruiser in perfect shape and good seals and didn't offroad it alot, I would love carpet. It gives you a nice clean look and a little sound damping. The bad thing is on rainy days you have water or snow on your boots and thats when you have trouble.
I used to use the indoor/outdoor carpeting from Home Depot, cost me Maybe $15 to do the entire interior. Was easily removed for cleaning. I liked how it would absorb a lot of dust, and hold it, until I 'cleaned' the interior on a windy day with the compressed air. This won't look as nice as a $150 formed carpet kit, but it's low risk to try at $15. What if you ultimately change your mind and don't like carpet?
Even though it rains in San Jose 7 month out of the year, I have had exposed metal not rust for years, in areas where it gets frequent contact, like from your shoes on the door opening. The original poster is in Cali, so depending on the exact location, there may not be much of a rust issue.
i herculined my tub and then went with the rubber Aussie mat that cool cruisers sells and really like it, its a little spendy but fairly well made and has held up well and i also bought the insulation kit they sell it helped quiet things down a lot. just a thought instead of carpet.
I work for a rug company. So we have lots of stuff laying around. I had the fabrication department fad me up out of some scrap pieces. No latex backing. 100% synthetic. It doesn't hold moisture. Works great for sound deadining and heat sheilding.