1781ccT

I'm updating my father's older '80's era Cannondale touring bike from downtube shifters to 9 speed STI.
It was originally 126mm spaced 6-speed in the rear, with a triple up front.


We've got a rear wheel with 9 speed cassette installed.
New-ish rear derailleur and rear shifter installed, and that works just fine.

Front shifter is installed, but I cannot get the original front derailleur to shift properly.
High/low limits are set properly, this was unchanged from the old setup. I did lower the FD so the outer plate of the FD is within 2mm of the large ring, and it's definitely lined up properly.
I can set the cable tension low and get it to shift back and forth between the inner ring and the middle ring - but the FD will not move enough to come close to moving the chain onto the big ring.
OR - I can set cable tension higher and go between middle ring and big ring perfectly, but no possibility of moving into the little ring.

With that said, I was able to find a *perfect* tension such that ON THE STAND, it will shift onto all three front rings, but with heavy leaning on the shift to get big ring, and only dropping the chain onto the little ring 75% of the time, which means that out on the road under real riding conditions, it will shift there less often. With this very precise tension setting, there is a LOT of chain rub when the chain is on the inner or outer ring.

SOOOOOOO.....
My questions are:
1. is this problem related to the different amount of cable pull between the downtube shifter and STI?
2. could the spacing between chainrings be different from the 6-speed era versus here in the new millenium? (this, to me, is very closely related to #1)
3. is this problem related to the movement ratio in the FD for the amount of cable pulled. (also closely related to #1)
Basically, I'm hoping that a new FD will solve the issue, but hoped somebody could shed light before throwing more and more money at it.