heater bed oversized too much?

, I ordered and received 310 x 310 120v 750 watt heater with 4 holes for my 3d printer. I also installed a SSR to power it. my bed is exactly 300mm x 300mm so the heater pad sticks out 5mm on all 4 sides. It clears the Z towers with plenty of clearance. My question is 1) is it safe to run like this? 2)will it have an adverse effect on heating the bed uniformly? and 3) can I trim the 5 mm off?

I would think it will be fine. To be honest the edges of the build plate normally suffer from not enough heat. Having the heater extend out past the edge would seem to be helpful to prevent uneven heating. Just don't burn yourself!
This is just me thinking logically, I have no expertise with doing this so take that with a gain of salt.
Good luck with your heater!