I'd have probably gone for something like a 78 and an 83 or 85 to give a bit of range round your stock 81.
You sure the 80 you've ordered is durango compatible?
And the durango cars (210/410) use 48DP pinions and spurs.
The original set up will be optimised round the 13.5 turn motor. Using similar size ratio with a 9.5 turn motor will have you miles out.
There are some charts to show optimum gear ratios for various motors, you might have to search for it (petitrc is a good resource for things like this)
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