Back in those days they rated at "gross hp", which is at the flywheel, no accessory load. Today we rate at "net hp", which takes accessory load into consideration. As for the comparison to the 428 (as well as other high performance engines), there was no need to play games with the rating by "bragging" low, a version of Detroit poker (also known as sand bagging), with the possible exception of the '61 drag/track version. If anything, on the more mundane engines they may have even over rated them a tad just for grins (or marketing reasons). The 335 hp engines were moderate "performance" engines that were probably pretty close to the "reality" of the time. Of course, there will be those who will swear they had one that produced way more than that, so who am I to spoil their memories.