Well I thought I'd be good for practice to complicate the build. File, fit, file, fit, braze, file, repeat. While I haven't put a chain yet, there is plenty of clearance. It's a 6 speed cassette on a 7sp freehub.
He's talking about the chain fit between the rear hub and crank...mixte's will have issues if the stay angle (viewed from the side as the stay goes from d/o to tt) is too shallow.
Fwiw, I too think this was an ambitious first frame- congrats for seeing it through, I'll bet you learned a lot! That said, I think you should fit the parts to see how things line up...and then go directly to frame #2. Maybe do a lot more fillet practicing in between.
Still, kudos for getting it done!