you look up the donor bikes speedo ratio and see if it matches yours. Same for the tach. You'll probably be in luck with the speedo as there are only a few standard ratios (I think all jap bikes are pretty much the same - 2240:60 - but I could be wrong). Speedos are never dean on accurate anyway. Tachs are a whole different ball game, the ratio depends on the engine and can be anything. cb750s and cb550s share a lot of small parts but they have different tach ratios and the gauges won't swap over. Your best bet is going with an electronic tach. You can find out if a tach is acurate by hooking it up and then getting a dwell meter that also measures rpm and checking it against that.
As for the fuel gauge that is going to take some fab because you are going to have to get the existing fuel sender to work in your old tank, this could be as easy as adding a bung to your tank or as hard as sectioning part of the old tank into the new one. Personally fuel gauges on motorcycles are for sissys, you wanna know how much fuel you have open the lid and look or clock it on your trip odometer.
So what gauge do you have in mind? Personally I have expirence retrofitting cb450 black bomber and cb77 superhawk headlights/speedos to cb750s, and I have done a yr1 headlight/speedo on an rd350.
As for the fuel gauge that is going to take some fab because you are going to have to get the existing fuel sender to work in your old tank, this could be as easy as adding a bung to your tank or as hard as sectioning part of the old tank into the new one. Personally fuel gauges on motorcycles are for sissys, you wanna know how much fuel you have open the lid and look or clock it on your trip odometer.
So what gauge do you have in mind? Personally I have expirence retrofitting cb450 black bomber and cb77 superhawk headlights/speedos to cb750s, and I have done a yr1 headlight/speedo on an rd350.