There are retrofit roller conversions, just a little more money and work then a factory setup. But from everything I have heard about the Mexican blocks, they are the second best production 302 block (Boss 302 being better, extreamly expensive these days, and nearly impossible to find)
The rumor story I remember from back in the day...
When Ford wanted to start maing 302s in Mexico they took some molds and got a couple of castings back. They were horrible. Researched what was wrong, molds were good, metal supply sucked. Project was now behind so fix it whatever it takes. Took the tooling in the back room and threw metal at it. It the cast iron wasn't as good, put more in to compensate. While at it, use the 289 HiPo main caps as well. All the yelling also made it back to the cast iron suppier (who was worried about losing a HUGE contract) started sending some quality iron to the foundery. So when production hit they had a beefed up casting and a good supply of iron, thus the better then most mexican 302 block.
How true that is, I have no idea. Just an old story from years ago.
Now for heads. Lots of quality aftermarket out there. Expensive? Not so much. If you consider a set of stock castings, rebuild with new parts, machine work, bigger valves, then find someone who can actually do a good port job and repeat the port job the same over 8 ports, you now have a set of stock heads pushed to the limit that come in about where a set of off the shelf aftermarket heads come in. And you are still dealing with cast iron. Aftermarket aluminium heads are the way to go, especially if you are after 300 HP.
Speaking of the 300 HP target, what is your target torque? Broncos are heavy, they love torque way more then HP. Torque is what makes them fun to drive. Torque is what moves you, HP is what gets you there. Moving is more fun then flying down the road at some speed faster then the chassis can handle. But I don't know your intended purpose of this rig either. If it is sand drags, you want HP. Fun to drive around, fun to play on trails, you want torque.
May I suggest a 351W???