GP100 and Security Six need advice

[Aggie01]

My dad has a 4" Stainless Security Six with factory target grips. He has been telling me that he wishes it had rubber grips and a 6" barrel for the better part of a year.

Well, it turns out I have a replacement GP100 coming from ruger for my cracked cylinder. The nice lady on the phone said they could probably replace with 4" barreled one with a 6" barreled one no problem.

I felt the GP100 was a little "beefy" and liked his Security Six as it is.

Any impartial observers think a straight across swap is a good deal for both parties?

Is there a half underlug 6" GP100 available, or is it full underlug only?
I'd like the model number, if it's a thing that exists.

Thanks