You could mount the red dot on the rib.
They make a no gunsmithing base for that.
Also make a lower cantilever base that attaches to rib, but would put the reddot closer to the eye (over receiver).
If your SP is a rifle sighted choke tube bbl, yeah, diff bbl or do receiver mount.
Since you are doing shot/ red dot sight..........have you looked at the Weaver clamshell mount?
It sits low, is plastic.
Non see through type.
They work, but IMHO have two problems with that mount.
1. The nuts fit in pockets and are sloppy. You will need to epoxy them in.
2. the width of the mount.........at the back it might make hand contact.....just a little clumsy.
Might be able to bevel here and there for less annoyance.
The Weaver mount is under 30 bucks. Epoxy the nuts in place and slap a Sig Romeo 5 on it (or 30mm Aimpoint clone like the one from Primary Arms).
The Sig will turn off and on automatically, so for a grab and go might be the better, but it's a micro sight.
I find them tolerable closer to the eye.
IMHO if not shooting slugs, no need to D&T receiver.
Bead sights .
I've tried the clam shell mounts . Not for me .
I'm going to buy the jig and D/T the receiver.
Just wondered how well the jig held up over time .
Guess I'll find out .