Way back when, I used that bulletin on the 2.6 as a starting point for modding the engine in a Fire Arrow I used to have.

That probably goes back to the 80's.

What Worked:

If you had the combo catalytic converter exhaust manifold, anything was an upgrade!

The factory tri-y exhaust manifold is pretty hard to beat. On the street, I never saw an improvement when I swapped to a 4 in 1 header.

A good exhaust from the last Y back can help.

Milling the head, seemed to work very well. I cut the heads up to .040" with good results (.025 was typical). I also reworked the valve bowls a bit to smooth out the sharp edge on the short side radius. No radical gain, but again it seemed to add a bit of snap.

Distributor advance springs from a Starion. Quicker initial advance, again a bit more response. I also upped the initial advance 5? to 7?.

Bigger cam (John Baker performance cam). Bad mod... No noticeable gain. Less bottom end, not enough gain on top to offset it.

Dual side drafts. Nice increase in response, some gain on top, horrible fuel economy and poor cold starting. An expensive experiment that didn't pan out.

Working secondary barrel on the stock carb. Best mod ever!!! The power loss from a non working secondary is amazing.

Upgrade downdraft: I never tried one, but the reports are generally positive and its a lot less complicated than the factory carb.

Roller cam from the AU motor. On my turbo 2.6 it added mid range TQ but is hard to find.

I fooled around with the NA 2.6 for four years. Small improvements can be made that add a bit of pep. The EFI system from Australian 2.6 would be a nice score. The down side is the water port on the head is different and it needs to be modded for a fore-aft motor layout.

Balance shaft elimination: There are some outrageous claims on HP gains from taking them out. I never saw a enough of a gain to make it worthwhile. That and I like having a smooth feeling four.