I know may sound like a dumb, but have you checked that the fans are pulling instead of pushing. But I had the same problem when I switched to mishimoto rad and slim fans. Always overheated after driving for a bit especially uphill. Could never figure it out till one day I read an article about...

Once I got my gsx running it's been my daily for everything . I came across it by shear luck. I've got a 1995 gsx and I did a 6 bolt swap all on my own and a ton of research .