I found that the best position to mount the speedo is with the motor / power leads facing the front of the car. This was you are able to see the LEDs when you are setting up the speedo.
The reciever / switch leads can then run down the side of the speedo, ie under the LEDs.
I then run motor leads and sensor lead through the middle of the rear upright. You can then sorten the motor leads to make a neat finish. I would leave the sensor lead alone.
Place the capacitor at the front of the speedo, along the edge of the battery tray.
Leave the heatsink on, but put the fan at the front of the speedo so it blows across the heat sink. The leads of the capacitor do a goo djob of holding the fan in place, but a little double sided would be usefull. With this layout, it will all fit under a standerd B4 shell.
Remember to reduce the overall rear ratio.
I ran indoors at Stoke with a 6.5 on 20/81. It was overgeared by one pinion but nothing overheated.
Hope this helps.
Phil.