Not real sure on nissan computer signals but I think the computer will be monitoring the voltages and not the resistance.
Sounds like Xf TPS = 0 - 10 volt and Nissan = 0 - 5 Volt absolute readings.
Both are industry standard signals and you can get simple convertors for the task of scaling these voltages, even a 0 - 5V convertor will easily be adjusted to give you 0 - 4V if that's what you really need...
If your even a little handy with a soldering Iron I think you can buy a build it yourself kit from Jaycar or similar prolly from around $30...
Hope this helps.