I did the conversion over twenty years ago - which means I am an old guy. The bolt pattern is exactly the same as the HS6. The stock manifold accepts the carbs and the heat sheilds fit too. You just have to port the manifold out ( 1/8 of an inch on the radius) to match the carb bore.
I sold the car, a 72 142S, one of the best I ever had, in 1984. The carbs went with it. Had IPD lowering springs, R-sport front bar, big bore exhaust, and 185/70-15 Pirellis on stock wheels. Went like hell, handled as well as my 79 242GT which I had bought new a few years earlier, but later sold to buy a boat......so I made a few bad choices in my life.....
The HIF6 carbs from the 142 I sold to fellow with a Lotus Europa JPS. I designed and cast a short aluminum manifold to be able to install the HIF's in place of the stock Strombergs. The little 1600 twin cam liked the stock B20 needles and it developed much more power. He still has the car, and I still drive it every now and then. I did that conversion twenty years ago too.
Been doing things like that all my life it seems. I now have a domain name - I am at www.sucarburetors.com