Quick answer… Unless there’s a compelling reason to not use the AM29, it sounds like a natural choice without knowing your situation on budget, price, condition etc. Also, if others are running the AM29 in the 206 class then it’s easier to compare notes too.
Longer answer: You’ll find people have been able to make almost any kind of chassis and axle size work. The four stroke specific chassis usually have two main features: 1) Lower price point, 2) More clearance between the right side. Beyond that there’s some differences sometimes like a 40mm axle vs 50, but again, like I say you’ll see both types at the front of fields.
The problem with grouping a chassis as “stiff” or “soft” based on frame rail size is that there are other factors that affect load transfer and spring, arguably more. Material, wall thickness and of course tube layout make a big difference too.
If you want to geek out, here’s a good topic on four stroke chassis (As much as I dislike the term 
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