That's the real problem. No one is telling you the specs. The mirror guys aren't and the pressure transducer people aren't.
In any event, here **broken link removed** is a pressure transducer, but it's WAY more expensive that Dynotune. Hopefully you can get Dynotune to send you specs of the transducer and get the mirror guys to tell you some of the inputs they can accept.
0-5 ratiometric is probably a "good input". zero can never be achieved and neither can 5V. So, you get something like 0.6 to whatever the power supply is minus a bit. so, at exactly 5V, the output may be 0.1, but if the sullpy was 5.1 V, the output would be different.
You also have the engineering units thing to deal with, If the thing only displayed 199.0 full scale, then you have to pick a gain and an offset to read 0-1999 PSI, for instance.
Because 0V and 5V are not obtainable with a 5V supply, ratiometric is good. You can get close to 0V and close to 5V, but generally it's very difficult to read a voltage close to the power supply rails.
It's really easy to get a uP to read a ratiometric signal.
So, I can see the probem, but not the solution when neither manufacturer talks to you.