If you want better single core performance, you still wouldn't want a 7350k and B250 motherboard. You'd want to overclock for as much single core performance as possible, so you'd need a Z series chipset motherboard, and an unlocked CPU. It also doesn't make sense to buy last gen parts, especially when they're the same price or more expensive than current gen.
So, if you were to go Intel, get a Z370/Z390 motherboard and an i3-8350k/9350k at least. Otherwise, you are just better off with Ryzen. An i3-8350k/9350k will have better single core performance than a 7350k, and has 4 real cores instead of 2 cores, 4 threads on the 7350k. However, an i5-8600k/9600k is only like $30-50 more than an 8350k/9350k, so it would make sense to just get the i5 IMO.
I'd still be going Ryzen though, especially if, as stated above you use software that will benefit more from Ryzen's cores.