Welcome to the forum

Go with a larger diameter pipe, it gives a lower tone, like a pipe organ, the larger the diameter the lower the note

And the straighter the better

Exhaust shop should be able to custom make what you need

 

 

From 1995 and up, Rangers had the tach wiring at the instrument cluster so you can swap in a cluster that has a tach and it will work

1995 to 2003 Ranger clusters will work in your 1997 Ranger
Also 1995 to 2001 Explorer clusters will work
4cyl, or V6 engine cluster doesn't matter, they are the same

I would replace all 6 backlight bulbs in the new cluster as preventative maintenance

1995 and up Rangers use the Crank sensor for RPMs, not the coil as older systems did, so tach signal comes from the computer and runs to the instrument cluster

Above is best way to get a tach in your year Ranger

You could try using a 3rd party Inductive Tachometer
This tach will have a sensor that clamps onto 1 spark plug wire, it reads the Inductive current when spark plug fires, Ford uses a Waste Spark system, so each spark plug fires once per 1 RPM, not every 2 RPM like most systems, but 3rd Party system should be able to be calibrated for that
And no, your dual spark plug setup doesn't change that, both spark plugs in each cylinder fire at the same time