I've found the rear sight bases a real bugger to drill for the indexing hole at TDC. The last time I did it on my little mill with centring bit, after milling a little flat, that went fine. I've got a couple a little wrong, and just deleted that srcew and ended up only using the cross pin to secure it. Not right but they worked fine. Next time I'll make an adequate drilling jig.
I'm sure you'll sort out a set of parts that work and fit well.
I love these sort of projects. They're not always a cost effective way to acquire a running rifle, but if you just went out and bought one that shot well off the bat, you'd miss out on learning about all the details about fit, and the relationship of parts, which I find very interesting.
Good luck with this build!