Just my thoughts..
How many honest 500 hp 340 engines are there you actually know of dyno tested/proved?
We know of many 340s @400-480 or so.
Some at 500 ish.
W/318 It can be done, you just have to get another 18ish hp somewhere ,up there, to make up for the 3.93 bore.
Possible, yes. A ton of them? Show me where.