Can't say much about darkhorse's, but the folks at crites are about as helpful as a deaf mute monkey. I've got their swap kit (it took 6 months just for them to get the headers to me) and got it all in, though the collecors are a real bastard. They have zero to nil tech support. I just talked to a girl today on the phone and she told me that if I wanted any tech support I was out of luck. Then she hung up. That was several hours ago and I'm still pissed. BTW, 460 in my 66 Fairlane does look gorgeous. Think I stared at it for four hours after I got it in. Beautiful.
From what I understand, Mr. Crites is helpful if you can get ahold of him. And the Mrs. is not. I have purchased some of their stuff and it is good, but don't expect much feedback. It's pretty much take your order, thank you, your stuff will get there when it get's there.