There are two issues you need to consider, the first is the age and condition of the existing harness and the various connectors. Has the harness had any alterations or add ons that you can see, also after 50 years some of these may now be getting a little fragile, any signs of melted cable anywhere, if so what is happening underneath. Only you can see the condition and need to make a judgement on that aspect.
Secondly, some of the wires are going to be a little stretched if you decide to use the existing harness, but yes you can always extend the cables, but make sure you use the correct wire sizes. I have seen numerous adaptations over the years where people have used any old wire and the same gauge wire for everything to extend their system. This could and no doubt has led to fires and other issues. Some use the same colour wire for every extension which later leads to confusion as to which wires are going where, so again you need to try and find cables of comparable colour codes or as near as possible. As you extend make up your own diagram as you go so you know later which cable is which.
A simple way of telling whether the cables will stretch is to use a piece of string, use this to measure where the cable currently exits the main part of the harness and stretch it across to where your new instrument/switch will be, then check that length against the existing wire to see if will be long enough to reach the new location.
If done carefully and using the correct wire gauges then you can certainly extend the system. I wonder whether it is worth the trouble given that a new dash harness is not too expensive set against the cost in time and money of buying the cable and connectors to extend. But then I am spending your money, so the decision has to be yours. The parts if you decide to extend will include,
- A pair of good wire strippers and crimpers
- A soldering iron (all joints need to be soldered , no scotch locks or terminal blocks)
- Shrink fit insulation to go over the new joints
- Cable of various sixes and colour codes
- A selection terminal connectors, both bullets, bullet rubber insulators, lucar spade connectors (there are two sizes from memory, one is approx. 1/4" and a smaller one approx. 3/16" both female and a small number of males)
- Harness tape which is different to insulation tape in that it sticks to itself without the adhesive that backs normal insulation tape. Insulation tape starts to unwrap over time. There is some sleeving on the market which you can use instead of harness tape which we use on our race cars, which expands as you run the wires through it, so it can take a number of cables and keep them all tidy, brilliant stuff, look here,
I hope this gives you a better idea of what is involved and helps decide which route is best for you