In addition to the parts and complications already mentioned, the frame may also need to be spread to accommodate a new index-compatible freewheel or cassette. It's probably spaced at 120 or 126 mm. It'll need to be 126 mm minimum for 7 speeds; 130 mm for 7
or more.
And does the bike have a derailleur hanger? I'm not sure I would trust a claw-mounted derailleur to be precise enough for indexed shifting, more so as the number of speeds increases.
You'd also need downtube cable stops for brifters. If the bike has downtube shifter bosses, you can mount stops on those with built-in cable adjusters to fine-tune your derailleurs' indexing. If not, you'll have to use clamp-on cable stops and inline adjusters.
Originally Posted by
Trakhak
You might have more of a problem with fitting the STI levers on the handlebars, which appear to be steel and thus incompatible with the clamping bands of the STI levers, which are designed for the larger diameter of standard aluminum handlebars.
If the bike was made in the late '70s, I'd think the bars are going to be 23.8 mm regardless of whether they're steel or aluminum. The two bikes from that era with steel bars I've had in my garage -- an '81 Fuji and a '72-73 Peugeot -- both had 23.8 mm bars. Of course, it wouldn't hurt to measure to be on the safe side.