I have a 1971 914 with fuel injection and points and I’m looking to install Pertronix electronic ignition. Do have to modify primary wiring to ensure 12 volts at positive side of the coil? Will the factory tachometer still work?

Just use the correct Pertronix coil for the module you buy ,I use Igniter II and Clewitt Eng plug leads for my "6",been great for some time ...no issues ,I don't use the Pertronix coil and I have Permatune CDI so use their matching transformer coil to best effect.
Your stock tacho should be fine,you will need to wire the setup as per their instructions however.Click to view attachment
Click to view attachment
Shown is the 3 pin CDI and the other for T4 motor.
If unsure you can visit their site and download the fitment manual before you order your unit.

Pertronix I and II will work with your OE coil. The Pertronix III will not work with OE fuel injection.

It is an incredibly simple two wire hook up. Just put positive to positive and negative to negative on the coil and you’re off to the races.

I love my Pertronix Ignitor II!

Why won't the III work with fuel injection? Do you mean specifically L-Jet, which takes its injection timing from the ignition points, or all fuel injection?

If you have a vacuum-advance distributor, make sure you get the version of the Ignitor that accommodates the vacuum arm. If you get the other one, the rotor will not sit down all the way on the shaft and you won't be able to put the cap on correctly.

--DD

You’ll have to talk to Pertronix, but they specifically said that the multi-spark on the Ignitor III is meant to work with carbs or throttle body injection.
I was advised not to purchase the three if I have OE fuel injection regardless of the year.