George, give your head a shake, a centermount progressive is absolutely the worst possible choice for induction on a flat 4 aircooled motor. The long intake runners aren't heated so they condense the fuel out of the fuel/air mixture on the way to the cylinders, liquid gas doesn't burn so you have to jet much richer than it really needs to maintain a burnable mixture, the liquid fuel just passes through the cylinders washing away the lube on the cylinder walls in the process. The result is terrible gas milage and premature engine wear. They also don't run well out of the box, you'll need to try a few jet changes before you even come close to the performance you get with the FI, count on spending an extra hundred or more on jets and farting around. There's also the issue of the way they behave in cool wet weather, count on engineering some sort of serious preheat for the aircleaner unless you enjoy stalling and hesitation. Great choice for a sandrail that runs full throttle in the hot desert, or an inept flipper who is too lazy or stupid to fix the FI and wants it to run good enough to sell, but friends don't let friends install a Progressive, it's like whacking your thumb with a hammer on purpose.
If you are bent on carbs spend the extra bucks for a set of duals, or better yet OG 72-74 Solex's, the extra cost will cancel out after a few tanks of gas and it'll run much better even though you'll likely still have to play with jetting.
That FI system put up with 40+ years of neglect before it started to act up, it'll cost less to fix it right and then it's ready for another 40 years, it's perfectly tuned for your engine right now and works flawlessly in any weather, if you don't understand carbs stick with the FI unless you are ready for a steep painful learning curve.
All parts are still available for L-jet, AFM's can be rebuilt, new harnesses made, injectors cleaned or replaced, and GeeBee makes all the oddball hoses. Dozens of other makes use L-jet as well so things like that CSV are also found on Nissan/Datsun, Isuzu, BMW, Lotus, etc..., whatever it costs you will be much less than new carbs and the time to set them up well. There's also the option of just deleting the CSV, it's not needed above ~0*C on a healthy engine.