the only problem you would run into is if your overflow did not have the gph to keep up with your pump. to be on the safe side you could of course ball valve it too, which will add a bit of head pressure, but will give you that margin of safety. going that route would still give you maximum power should your overflow be able to handle it however.
i suspect that your overflow is at least 500 gph after your modifications, probably closer to 550-600. you can do some basic measurements and estimate your head pressure, and that will give you an idea of wether or not your mag7 will go over that.
if i was in your situation, i would step up to a mag 9.5 and make myself a nice spraybar. the extra turns and nozzles in the spraybar will more than absorb the extra flow, and with a ball valve you are 100% safe. mostly because i wouldnt want to rebuy down the line... i am cheap and like to try to do things once.
whichever will give you great results though. you can always do it as planned, and if you need more flow, you can do a closed loop system, or rely on in tank powerheads. if you have already done some shopping, or are more confident with what you have been working on, that might be a much better way to go. no reason to risk an overflow if you are close to your maximum overflow rate already.
HTH