TromboneChat

I am returning after 40 years and about 60 pounds. I'm finding my neck is kinda pushing the mouthpiece a little cockeyed on my lips. I play a King 3B(f) from 1980ish. I would love to get a new (larger bore) horn after I get my chops back, I played a 42G (rose gold bell?) in college. Do larger bore horns or more modern horns have more "neck room" for less than petite people like me?

Yes, larger bore horns have a wider "reach" than your 3BF.

As someone familiar with your problem, I also found that opening the dihedral angle (between the plane of the bell section and the plane of the slide section) can also help with the fit.

Yes, that is definitely a thing. The distance between tubes in the handslide is a bit wider on "large" horns. I don't have any idea why this is a convention.

I have always had a larger than average size neck, and I had a "small bore" slide custom-built with a wider crook (so the tubes can be mounted wider apart) and that was a noticeable improvement for me. Other people have tried using a mouthpiece with a "bent" receiver with success too. I actually like how the change in crook makes the horn blow, so for me that is a better solution.

There is a nice survey of slide widths in this other thread:
viewtopic.php?t=18492

Wycliffe Gordon is a big guy. Look at how he gets his trombone to fit.