Not YB specific, but 84mm isnt a long stroke by any means. I cant comment on how that might fit in a YB block though. I'm sure you could clearance some areas for rod clearance.

I'm certainly no expert, but in my opinion steel will be the strongest, and probably the best option. And also the most accessible as plenty of places will make steel rods.

I'm sure if you asked any of the above people, told them your application, piston weight, crank details and intended rpm, they could advise what is best as they'll be able to calculate the various loads.