I've been doing it for 50 years. It is easy and not a secret. See post #17, #48, and #56. What was factory 340 hp, 275? What times did they run, low 14's? Looks like it is doable... I used to take a pair of milled rebuilt 360 heads, drop a 340 cam, springs, double roller timing set, high pressure oil pump relief spring, a windage tray, a hardened tip oil pump shaft, 340/360 4 barrel intake and rebuilt 340 carb that others were swapping for a slower combination, a Direct Connection electronic conversion kit for around $100, and 360 or 340 exhaust manifolds with dual exhaust. You think that combination would not run with a 340? And what's more, it works with a 273 or 360. Add a custom cam and add "64-67" 273 rockers, and you are now faster than a 340. One can do a lot better than stock 318 performance with just a different cam and valve springs with no dependability problems.