Hi Neil,
I have been trying to get a result with your posted model that matches the Blue Head profile.
The blend option didn't work so good so I modified your red Head profile and then ran the lofted
neck into it. Boolean union neck head and base. Filleted the neck to head with 1"radii.
The neck and base joint was filleted 0.14", this was so I maintained the base profile as Boolean
subtracting which cuts thro a larger fillet would affect the profile.
Cheers
Barry