THIS is a roundabout way but the anime episodes are #46 The Decision, 47 Partnership, 48 Party boy, 49 Just You Wait, 50 Last Inning, and the game ends in 51 Seek Diamonds.
Find similar names in the manga chapter names - there is a chapter: Partnership ch 396, it's one of the chapters about Seido vs Yakushi, which is the game Seido won that sent them to Koshien, the Nationals. That game starts around chapter 383 and spans all the way up through ch396 Partnership and on up to chapter 401. That should put you in the ballpark - haha -for finding it. Hope this is what you're looking for.
Chapter 412.1 has a bit of the last few plays , starting on p14. Chapter 412 shows them at Koshien.
Unfortunately, the last chapter is missing, the one where they win against Yakushi. There's just that snippet in 412.1
I need help. Which chapter does it show Seidou won the Fall Tournament? I am confused with the chapters.