Best next upgrade?

First, NICE car. I am rather partial to 69 Fastbacks. I have had mine for over 30 years now.
I would DEFINITELY do a front disc brake conversion first. I think my disc conversion was the best money I spent. While you are doing that, rip out all of the front suspension and get a front end kit. @PST is a great option, because they are forum sponsors and offer a FABO discount. If you car does not have a front sway bar, install one. I did and it made a huge difference.
While doing all this, it is up to you whether you upgrade to LBP wheels.