The 351 is about an inch wider than the 302 due to the taller deck height. A shorty water pump helps, but the SBF is still going to be tight, and the 351 will be tighter. If it were mine I'd strongly consider stretching the frame a few inches instead of setting it back in the firewall. The cockpit is cramped enough without pushing the engine back, and if done right the longer hood actually looks good. People will see a difference, but will be wondering what.