The sentence in italics below was added 3/11/2012. The penultimate paragraph was expanded on 3/15/2012
I'm on my second 337 (a G model without aux tanks) and they've always has fuel gauge issues. On my current one, after lots of work, I've given up, relying on a calibrated stick before each flight, VERY careful flight planning, and keeping track of both current and recent fuel consumption (i.e., gal per hour at various operating conditions).
Let me add that most models have an electronic box (typically on the roof behind the headliner) that takes the inputs from the tank sensors and generates a signal for the gauges. Therefore, I don't think that using the old sensors with a new electronic fuel gauge will work.
Let me add that with separate main and aux tanks, there are techniques for reliably managing fuel on long trips -- perhaps even extending range. My first 337 was a 337D with aux tanks and I used some of the techniques to fly from Miami, FL, over the Rocky Mountains, to Boise, ID. These techniques depart from POH procedures and should be used only in special cases and by knowledgeable pilots who are very familiar with the aircraft's fuel system (the "Fuel Supply Management" page on my "backup" website
www.SkymasterUS.com provides such familiarity).
I'll describe one of them for reference only. You should use it ONLY if you independently determine that it is safe to do so. As a minimum, it assumes that you know with certainty that one engine (say, the rear) consistently consumes more than the other. On a long mission you start with all tanks full (which means YOU did it or visually checked). Run on mains until the rear engine starts stumbling because the right main tank is depleted. Immediately switch the front (non-stumbling) engine to its aux tank. After confirming front engine is running fine from aux tank, switch rear engine fuel selector valve to cross-feed from the left main tank, using the electric pump to initiate flow and restart rear engine (you know the left main has fuel because a) the front engine was drawing from that tank only moments ago, and b) fuel from the left aux tank is now flowing INTO the left main due to the design of the system, where return fuel always goes to the main).
After the rear engine is working flawlessly on crossfeed (about a minute) switch the rear engine to the aux tank.
While there are risks and trade-offs, covered in the next paragraph, there are several advantages. First, you will know PRECISELY what your consumption has been today (based on the time and gallons used to main-tank exhaustion). Second, you know exactly how much fuel remains (the contents of both aux tanks), so, third, you can determine exactly what your remaining range is (say you were using 9 gal/hr per engine and you have 36 usable fuel in the 2 aux tanks and you want a 30-min margin, so you still have 1.5 hours to landing).
There are risks and trade-offs The risk is that if you miscalculate and have very bad luck, both engines will die, you won't be able to draw from the full aux tanks and you'll be on a glider. If you are CERTAIN, however, that one engine (doesn't really matter which) has greater fuel consumption, then one will run out of fuel many minutes before the other one, and this works well. (BTW, even if on THIS trip the consumption was nearly identical for both engines, what are the chances that both engines will run out at EXACTLY the same time? Even if they are 5 seconds apart, this works, provided you're ready, do it right, and do it quickly.) The trade-off is that there are CRITICAL fuel management tasks ahead. In roughly 60 minutes the aux tanks will run dry, perhaps at nearly the same time, and you will need to switch to the mains using the aux electric pumps (to access the roughly 60 minutes of additional fuel that was initially in the aux tanks but was transferred to the mains by the return fuel/vapor lines). The reason for the word CRITICAL is that you don't want the tank swap while you are low and slow in the approach or pattern of the airport, and notice that if you adhere to IFR fuel reserves of one hour, that is exactly when it will happen. So, if you're landing 30 minutes after going to the aux tanks, you will be OK but if it's much after that, then the swap needs to occur while at altitude before getting low and slow. A rough rule of thumb is that the fuel in the main tank after you switch back to it will last as long as the time that you ran on the aux tank (e.g., if you ran 30 minutes on the aux, then you have 30 minutes of fuel available on the main). You can see, considering these risk and procedural issues, that the approach described here should be done as a rare exception for trips that are at the outer edge of the airplane's range and only by a competent, experienced pilot who knows the fuel system thoroughly.
While repetitive, I need to stress again that this is for reference and should not be used except in special cases, such as an aircraft with appreciable different consumption between the front and rear engine, and then only after careful analysis and preparation.
Ernie