I strongly suggest you don't mess with the hammer sear engagement. Unless you know what you're doing, it's easy to ruin it or even make it dangerous. My '73 has basically the same hammer trigger arrangement as your '66. I bought it used and the previous owner had done a chop job on the trigger and ruined it. It was unsafe and I had to replace he trigger and hammer.
Reducing the strength of the hammer spring will have an effect on the trigger pull by reducing the amount of friction the trigger has to overcome to release the hammer.
You won't have much luck cutting a slot in your spring, those are done before the spring has been hardened and tempered. You won't have much luck cutting through the hard steel and controlling the cut.
Putting a shim of some sort under the spring will help some. More effenctive is regrinding the spring to a 'wasp waist' shape, or hourglass shape. If you're very carefull, and go slow, you can do this yourself with a Dremel tool and the smallest grinding wheel you have for it.
A few cautions: Hold the spring in your hand while grinding so you can feel how hot the spring gets. Dunk it in water after every stroke to keep it cool. If it gets too hot to hold you will probably ruin the temper, but you can keep it cool by holding it in your hand. Keep all grinding marks along the length of the spring, do not go across the width or thickness. Most Uberti springs have some tooling marks across the face. Theses are excellent stress risers for the spring to break at. The original spring on my '73 snapped in half halfway through a match one day, right on one of those tooling marks. Carefully grind them out with your Dremel tool, working along the length of the faces so that the new tooling marks you leave behind are lengthwise, not across the spring.
Lastly, when you are all done, go over all the edges of the spring with the finest files and stones you have to round off and relieve any sharp edges or burrs left behind. Sharp edges and burrs are places that the spring wants to break at when stressed.
I suggest you order a spare spring from VTI or somebody to work on, so that in case you go too far you can still put the original spring back in.
Go slowly and keep trying your results until you achieve the hammer stroke and trigger pull you want.