starter or Ignition trouble

[Mrcustom]

I just finished hooking everything up in my 57 pickup brand new crate 350 motor, brand new TH 350 Transmission, Radiator, well you get my point. So I start it up sounds great 75 lb oil pressure 14.7 amps 175º .Sounds good now the problem is if I turn the key to off quick all the way it sounds like the starter is engaging but if I only turn the key half way off, I guess it would be the Accessory. The motor shuts off fine no problem.Very annoying and it kinds of scares you when you don't expect it. Any Ideas? O yea with the old 400 that was in there I didn't have this problem. I did though have the problem where I had to let go of key to get it to start. Hope you can understand my babbling.

Jim