There’s a lot of guessing in this thread.
I’ve run both turbos on a 360. Do NOT put a 7875 on your 318. If you already made the purchase I’d urge you to return. It will be laggy as hell, and you will not be happy. It will be laggy on a 360 also. The cheap(< $300) gt45’s are ALL 69mm. Every. Single. One. So don’t stress that aspect. Get the $200 one. A gt45 will work fine and you’ll be happy with it on any of your future engine options. You won’t need anything bigger until you build every bit of your drivetrain. The 50mm wastegate will work great. Don’t go smaller or you may have trouble keeping boost down(<10psi) unless your plumbing is PERFECT. If you go cheap on your wastegate which I’d advise you not to do…but I did; disassemble, check for defects and debris, and use valve grinding compound to ensure you get a good leak free seat.
Blow through carbs can be hell. If you’re building yours I have some free parts for you so pm me. For your sake I hope you’re buying a nice blow through prepped unit. Either way, you’ll have to be extremely careful with those cast pistons. I would recommend never running pump fuel even if you think you have the tune perfect.
Lastly. You’re going to want a nice converter for any of these combinations. Get one sooner than later, if funds allow, and you can have it re-stalled down the road if need be.