It took a while (just a lot of other things going on), but I did the following:
1) Installed M.2 drive. Had to purchase a standoff with internal screw online. I know I could get a bunch of them cheaper, but this worked: https://www.amazon.com/gp/product/B06XBG1783
2) Used Macrium Reflect to image the boot drive onto the SSD (resized primary partition so it fit)
3) Power off, disconnect internal hard drive, power on. Booted from M.2 SSD without any changes.
4) Power off, reconnect internal hard drive, power on. Still booted from M.2 SSD (Hurray!)
5) Used Macrium Reflect to image M.2 to a portable hard drive as a backup.
6) Installed an additional 2.5 inch 2 TB drive. The inside of the case (front side) is set up for two 2.5 inch drives with holes predrilled. Used the vertical install point, attached with two M3x4mm (might have been 3mm) screws and straight connector SATA power and data cables. It's a nice touch that all of the holes in the case are labeled with the screw size that they take (not the depth though.)
There is a horizontal orientation 2.5 inch mount point near the bottom of the case. That mount point doesn't work with either straight or "right angle" connectors because it's too close to the side (removable side of case). There isn't room for the straight connectors, and the "right angle" connectors would turn out through the front.
The 2.5 inch drive only mounts in that one orientation (the openings in the drive for the flanges on the case and the screw holes only align one way.) I have a "left angle" (or 270 degree) data cable, and am waiting for a "left angle" adapter for the power cable. Once I have that, I'll see if I can mount a second 2.5 inch drive.
I used this Cablecc 90 Degree Right Angled SATA 22Pin 7+15 Male to SATA 22P Female Extension Convertor Adapter with great results in all three of my 3670 PCs. For that lower 2.5" drive location.
I didn't want to use the option pictured a few posts up where a straight connector was attached. Since it was difficult for me to remove it. The angled connector worked best for me. So I got them to use in all three of my 3670's.
how to did the power cable and data cable for the SSD vertical mount? My iinspiron i3670 does not have spare cables?
I got mine from Amazon.
I got this for power. Which connects to the Mini 6 pin ATX port on the Motherboard and provided two more power connections
Then I got this 90 degree right angle adapter to put on the hard drive.
And then I got some sata cables.
i only needed the right angle adapter for the lower, vertical hard drive mount.
I used this hardware with all three of the 3670 PCs I got. Since I used both of the vertical, 2.5" hard drive locations. In addition to the 3.5" hard drive location.