Did the 65 and 67. A worthy upgrade imop. Lets you upgrade all the connections, grounds and add fusible links as well at the same time. The choice of alternators seems to be greater with the 12 than 10SI

As I'm not adverse to having these kind of upgrades visible, I'd not think of these as butcher jobs, but I do get the appeal of keeping it hidden. On the 65 I used a Amazon marine buss bar for as the body tie in and the Alt sensing point. Added headlamp relays with integral fuses up next to it. moved the horn relay down beside the washer jug. Eliminated the goofy rad support wire channel at the same time.

I like the look of the newer sealed fan on this 12. I think they are far more efficient than the pinwheel type anyway. What is wrong with a dash of modern I say.

94 amp 12S1 from a 84 Buick Rivera (I think) gave the correct clocking of the plug at 9 O'clock, top bolt at 11.

Did similar to the 67 when I  upgraded to the GMPP HO350, this was a Summit main PowerPoint. A mid 80's Camaro Alt I think. Littlefuse's from the bus to the lamp relays. 

Years in with zero issues. The BB addition that came after also uses the 12SI.

Even after all these years, this is still probably the best info available on line for the conversion,

 http://www.madelectrical.com/electrical-tech.shtml