Hi Maxheadroom,
the problem of adjustable speed has already been taken into account. You might place the pot off board and connect as shown in the schematic and PCB layout. If the speed is too high using a 10K pot replace it with a 25K pot.
LEDs used are 5mm.
Placing the LEDs off board shouldn't be a problem. Just use the vacant soldering pads of the onboard LEDs to connect the LEDs via cable.
Using an 8 wire ribbon cable connect two wires to +12V and the cathodes of the LEDs to the appropriate solder pads.
Don't mix up the sequence of LEDs since you won't get a "Knight rider" display in that case.
You might replace the 10KΩ resistor (R9) with a wire jump. The CD4017 can take full positive output pulses of the NE555 without damage.
C6 (6.8nF) and R10 (100KΩ) make a power up reset for the CD4017. If you don't require a power up reset omit C6 and use a 10KΩ resistor for R10 or connect pin directly to circuit ground using a wire jump.
If you want the PCB layout (Eagle) PM me your name and email address.
You might omit the ground fill on the PCB, but I recommend to use it for the reasons of saved etchant and less copper to poison our environment.
My inbox has been emptied in the meanwhile.
Regards
Boncuk
P.S. Send your accidently ordered NE555 chips to me.
I just can get DIP packages here.