The stock seven bearing engine has enough strength for three times the stock power, even with cast rods. Cast rods will break after some cycles, but that's about the only major risk.
Unlike Detroit V8's and V6's, there is no need for forged pistons because the balance and compression and rev range isn't in the same leage as a Z28 302 or Boss 302, so good cast pistons will do everything you need in a 250 to 300 hp engine.
Con rods should be replaced with forged items if you can get them. They are common and easy to change. They suffer fatigue resistence problems if reused and repeatedly leaned on, while the stock forged rods were fine to use
Asside from that, the only problems are rod bolts, which tend to be plastercine under loads, and the stock oil pump should be replace with a good replacement of stock or better than stock flow, with a new oil pickup and new anodised screen. And timing chains. Stock early 250 is fine, twin row aftermarket 200 timming chain is hard to beat.
Stock pistons are geared around compression and what works for a 255 V8 or HSC 2.3 or 2.5 will be strong enough for three times the stock rating in an I6. The 125 gross 200 is about an 85 hp engine (85 to 93 hp factory net). The 155 hp gross 250 was a 98 hp engine (or thereabouts). So that's 255 hp will have no main bearing issues on a formerly 85hp. And 300 hp likewise no issues on a formerly 100 hp engine.