I use "classiccarleds.co.uk". Not the cheapest, but very good quality, and they provide the dimensions of their bulbs. If you have a problem, when you phone them a real knowledgeable human being answers! Definitely worth the outlay to preserve ancient wiring etc.
Just as an aside, bearing in mind that I am not an electrician, when I fitted LED's to the 400, all was light and joy, but for my sins I also own a JDM GB250 Clubman which also received the LED treatment. Did not realise that as this model has only one repeater lamp in the speedo for the indicators, you have to use small diodes to stop all the bulbs flashing together!! Apparently, with filament bulbs, if you are signalling left for example, the current going to the repeater bulb is earthed through the right hand indicator system, but does not retain enough power to illuminate the right hand bulbs. LED's need only a minute amount to work so are illuminated. Hence the need for diodes, (one way valves!). The small digital fisher units are reasonably priced, and have a fixed flash rate no matter which bulbs you use, you can even mix them. Have fun, Mike.