how interchangable?

I have a 82 XJ750R Seca, came with a A/M speedo but no tach. I was looking on ebay, will a 81 XJ550 Seca gauge kit work with mine? or even a 750 Maxim's? will the fuel gauge work from the xj550's gauge on my 750 tank?

 

Welcome Thexad, the 550 cluster can be adapted to work on your system but you will lose the monitoring system and you will have to perform some surgery on the harness to get all of the connections in place. I do not know if the senders on the 550 and 750 are compatible.
The 750 Maxim gauges should work just fine provided they are for 82-83. You will only need to add a diode to your existing harness to get them to work without the neutral light being on every time you pull in the clutch. On a side note, this is one of my favorite mods for my Secas. The Maxim cluster is shorter and less obtrusive.
Could you please post your general location in your avatar? You might be surprised to find help could be close by.

 

Would it be possible to fit the the tank and clocks from a 750 maxim to a 650 maxim 1981 and what would it involve please :?

 

They would be a drop in physically Richie, the problem would be the wiring involved with the upgrades the 750 has. The 750 Maxim uses an electronic tach that you would have to install a wire from the coil input (grey wire) to the 750 cluster. This assumes you are using the 750K (us model). If you are using the UK model, you would have much more work to do. Of course I'm assuming that you want to utilise the upgrades. If you don't, your task is much easier.
I personally enjoy the challange but this would be a bit of work for the uninitiated. I would be happy to assist you in this effort provided you can send detailed photos and use Skype.