Hello there, lothbroki!
For the S340 and the gpu cooler type that you have (open-air style), a front mounted aio is objectively best. Here's a video explaining the different cpu cooler/gpu configurations, along with tests showing the thermal results:
A lot of ppl seem surprised about the results because common sense says not to blow hot radiator air into your case. But as the results show, what's actually happening isn't nearly as dramatic as our mind imagines. Evidence outweighs common sense.
I was in a similar situation as you, looking to possibly buy an aio for my S340 rig. While many ppl seem to be happy with Corsair's and NZXT's offerings, I wanted something serviceable... I like changing out the liquid and being able to make sure everything is in good order every few months. That's just me being an enthusiast..and anal lol. So, I went with the "be quiet! Silent Loop 120mm"--which has removable fittings--then replaced the radiator with a 280mm one from Hardware Labs (had best reviews at the time of purchase), 2 Cougar Vortex 140mm fans, PrimoChill clear tubing, and 4 nice 45-degree Alphacool compression fittings.
All-in-all, with the S340 and an open-air style gpu, I definitely think a front mounted water-cooler is the way to go if you want to improve your thermals. You could do a full custom loop or get an aio. My choice was kind of in between and I'm very happy with it. But aios today seem to be very well received and are cake to set up, so you can't really go wrong. I hope everything turns out great! Take care!
EDIT: Oh, and my cpu temps are 31C idle and never over 68C under load. My gpu (an old GTX 760) runs at 35C idle, and up to 70C under load.