the plate goes behind the carb
No it in front of the carb .Isn't that the way it is in thr picture?
Yes, you will want a bigger jet than you were running without the restrictor, I believe purple calls for a 38 but I'm not positive its been a few years since we ran a purple plate. I think ARC has some info on their website and you can always call them or Dover both are really helpful and have great restrictor plate carbs.Make sure that carb is setup for and jetted for a purple plate, they aren't just bolt on and go...