A lot will depend on what the PO may have done to run a positive earth. Here is a list of what they do with original positive earth cars for conversion to negative. You may or may not need to do everything and I suggest you have a good wiring diagram and verify if things like the tach are original or replaced with something different.

1.First you must change your battery cables or clamps so they will fit on the opposite terminals.
2.Leaving them disconnected, now reverse the positions of the two wires connected to your ignition coil.
3.Next, the generator must be re-polarised. Disconnect both generator leads. Take a piece ot 14 gauge wire and attach it to the main "hot" terminal of your starter solenoid or to a "hot" terminal on your fuse block. Reconnect your batteries in the new configuration (with the negative post going to ground).
4.Now take the loose end of your 14 gauge wire and flash it once or twice against the field (smaller) terminal of the generator, just enough so you can see a small spark. (Do not connect this wire to the generator, even for a few seconds)
5.Now remove the 14 gauge wire, and reconnect your generator leads.
6.If your car has an ammeter or voltmeter, simply reverse their leads.
7.Wiper and blower motors need no changes.
8.Original pre-1968 SU fuel pumps are not polarity sensitive, but modern SU replacements are, and must be replaced with negative ground SU pumps.

The tach will depend on what may or may not have been done to it, but this is a general idea for a Smiths type tach if they did indeed change it to positive earth or replace it with one.

To convert the tachometer to negative ground, John Twist offers the following advice:

Two changes are necessary to completely convert your early positive ground electric tach:

A) The wires must be reversed at the "white wire loop" at the back of the unit
B) The power and earth connections must be reversed inside the case

Step A: The wire in the "white wire loop" comes from the key switch and travels to the hot side of the of the coil. Referring to the illustration below, select one of the wires and tag it with two pieces for identification. Then, cut the wire between the pieces of tape, and cut the other wire to the same length. Reverse the connection (now there is one piece of tape on each wire) and splice (lineman’s splice) and solder them (remember, this is the power lead for the coil and is unfused). You may want to use a piece of shrink-wrap tubing to completely seal the new connections and prevent corrosion. When later replacing the plastic block on the back of the tach, ensure that the metal band around the block is carefully positioned. This is a necessary part of the electromagnetic pickup.

Step B: To reverse the power wire and earth wire inside the unit, it is necessary to remove the chrome ring, the glass face and the glare shroud. The chrome ring is usually removed with great difficulty by rotating until the tabs can fit through the slots in the case. Remove the two screws on the back of the unit that hold the internals to the case (not the two whose heads fit in holes in the case), and allow those internals to drop carefully into your hand. Don’t bend the needle! The spade terminal is the power connection. Just next to this is the earth connection. A resistor is soldered to one of these connections, and a green wire to the other. Unsolder these ends of the green wire and the resistor from their current positions. Resolder the green wire to where the resistor was connected, and the resistor to where the green wire was connected. Reassemble the unit after cleaning the glass.