The 300R tank is plastic as I see it, and other than the rear hold down strap vs a bolt fabrication that may be required, it'd make a nice weight-reducing swap if a stocker could be gotten at a good price?

I see some ads for the larger Acerbis tanks list the tank as fitting both models

 

Not sure how much transfers here on the Acerbis and OEM 300 tank, but on the front saddle mount bolt, I've quit worrying about it. I don't even try to hook the saddle into that wide head bolt. In fact I now use a short, narrow head bolt at the tank and don't even worry whether the slot in the saddle matches the tank. This on an '06 KLX. Hardly any of my off road racing bikes in the 70's or 80's had any connection between the tank and saddle. I think it's unnecessary.

 

When I first got my bike, I wanted to switch to the 300R tank for the extra fuel capacity and weight loss, but just eyeballing the tank I think you'd need the 300 shrouds & petcock. I wasn't sure if the 300 shrouds would fit over the 250S fans and didn't want to find out the hard way that they didn't.

I wound up going with the Acerbis instead. The Acerbis does fit fine, but in my case the greens are a few shades off.

 

I put together 1999 300klx frame with all 2007 klx250s parts, because I couldnt find a reasonable buy on 2007 frame. It will fit. I have a 2009 tank on mine now and had to use 09 shrouds and weld on tabs for rear mount of shroud.

 

Good point on the fan issue. I love the Acerbis tank, and it's guaranteed to clear the fan and other issues on the 250S.