I recommend the 100-280 range. The other one is for a different application and is practically useless IMO. My EV gauge was 140-320 and it hardly ever moved. I made matters worse by installing the sensor in the pan so most of the time the gauge was at it's lowest reading.

I later changed to an EV2 with the sensor in the test port and my readings are fairly close to what the PCM sees from the internal sensor so I like it much better. The gauge seems more responsive as well.

I did not buy the harness because I already had a tap off the dimmer wire for my other gauges. I still have the flickering issue at lower dimmer settings. You might want to call Clay at Riffraff and see what the harness is for exactly. Maybe it resolves the dimmer issue?
I keep my EV2 and my dash gauges at full brightness anyway so it's a non-issue for me. I really like how the EV2s light up at night. Very easy to read and not blinding.

I have no clue how the factory gauges work. I don't have an OEM trans gauge, I suppose they were added in the later years? I know my engine coolant gauge is a rough general idea at best. My volt gauge is even worse. I've had dead batteries and volt gauge still shows ok.
I have pretty much given up on my factory gauges for any sort of accuracy. I call them 'indicators' now because they don't really measure too well but they indicate ok.

 

You will want the R72022 so that you can match the lighting to your dash.

 

Now I found this gauge:
https://www.dieselpowerproducts.com/...r-options.aspx

It seems to better match the cluster than the Isspro. Anyone used these?