If built right i can personally attest for a 4.3 handling 300+hp without adding the PSI factor (i.e. Turbo/super charger). From what i KNOW he had in it was as follows, 98+ heads (ground for bigger valves and cleared for .500 lift), forged pistons/rods, balanced/hardened crank, comp cams custom grind (you would have to talk more with comp about what you want and such), Walbro 255lph fuel pump with helper pump on fuel rail too, eldelbrock intake mani with tbi adapter and a 454 tbi with 350 injectors, custom pcm ofcoarse, shorty headers with 2.5 true duals with x-pipe going into thrush glasspacks, high volume oil pump, clevite 77 bearings (main, rods, and cam). And i'm sure i'm forgetting some. o and he runs Royal Purple 5w-30 lol. But he is running high 13s in the quarter in a stock bodied 4wd 4door 1g Blazer, which with him in it is roughly 4100lbs, so iirc that comes out to about 350hp or so at the wheels. Now you wouldn't have to go all out like that to get just 300hp. But i am pretty sure with just heads, cam, intake, exhaust, and a tune you can get close to 300 without a turbo. And with all he has done he still gets about 17mpg city and 25 hwy.
Also if you wanted to stray from the forum i know someone on another forum that built a N/A 300+hp 4.3 that you could get advice from.