Sending units are different between analog and digital............but you can interchange them. There's a difference in the ohms so when you substitute one for the other it will either read a tad bit low or high. New senders are discontinued. And nobody makes a new replacement unit either. What I've done in the past is to modify the sender from a 79-85 DFI ElDorado or Seville. And those cars are either analog or digital too. Depending on the year all you might have to do is swap the fuel pump and change the electrical connector on the end. The SFI Riviera also uses a baffled fuel tank like the TRs. If you need a new tank get one for 79-85 ElDorado with DFI if they don't have it listed for a Riv with FI.
Eric, I may have asked you this a long time ago but going to again, are the 84-85 Turbo Riviera cams from the older carbed turbo cars? and if so, are they pretty mild compared to a stock 87GN cam? Another question, do you know the #part number of the stock 87GN cam? would it be #25517480 by any chance? Looking to replace stock cam in 85 T Riv with stock GN cam and lifters, not looking to make a race car, just a driver, have upgraded the intake, TB, turbo, ECM, coil pack, IC, and electric fan all to 87GN, just need to bolt it all together and cam needs minimum a new timing chain and or just replaced(100,000 plus miles).
Hope to make drivable for spring will post pics soon of installation, was able to retain all hot air accessories, headers, turbo position, downpipe etc.