911 e brake conversion, 65-69 backing plates Options

[Shivers]

I'm sure you guys are sick of this subject but I am in the throws of it. I've been reading everything I could find here on the subject and it is pretty straight forward. But everywhere I read it says to use the 1970 through 1973 911 plates. Funny how those plates are near 250.00 - 300.00 now. For one. I couldn't do that. So I went to two different places and got two 1965 - 1969 e brake plates for 110.00 and that included shipping. In the picture it looks the same, maybe I would need to do a little work on it, maybe a lot but my labor is free to me.
I received one last night so at 4:00am I'm making coffee and measuring the hole size of the plate and then the bearing carrier on the trailing arm. Both were 4 1/4" so I tried to slip the plate on and it only went so far. Well it took me a second but I was trying to put a square peg in a round hole. So I dropped the step on the side of the screw bosses 1/2" - 5/8" and radiused the screw bosses. I also added a radius at the front of the boss because the plate in the hole has a radius where the carrier is to mate with the plate. Got that done and the plate slipped right on to the carrier. I checked that the plate would fit inside my disk correctly and that it would also fit right when it had a hub in place with the disk. It does.
So here is my question for you all, what kind of boogieman is going to jump out when I least expect it. So far it is smooth sailing but I really like a plan for everything, especially potential problems. Anyone know what I need to watch for? Thanks