As Sweetpea said - you can make some simple measurements, make the standoffs to suit and if necessary simply make a hole in the firewall if required.

Another alternative is that you can move the firewall back to suit the additional length. It depends on the airframe. Many ARF's on the market are pretty weak in the firewall area anyway and it can be a simple job with simple tools and a little patience and care to to remove it and locate it further back.

It depends a lot on how much the carb protrudes into the firewall and how much "meat" is left between the engine mounts and the carb hole. I recently had the experience on this exact dilemna on a 27% plane where if I cut the hole out to suit the carb, the high speed needle would have ended up in line with the firewall and one standoff would have been very close to the cutout for the carb. an hour or so later, I had the firewall removed, moved back and epoxied into place so I didn't need to have the hassles associated with the carb in the firewall.