I have the same set up as you (t18 dana20) and looked at doing the same thing. You can get an adapter from advanced adapter (I think novak has it too)to adapt the new(er) dana 300 to the T18, but it is pricey ($500ish). The other thing to consider is that the dana 300 is longer than the dana 20 so you will have to shorten your rear drive shaft and have a longer front drive shaft.
To me, it was not worth it. I wanted the slower low range of the dana 300. The rear shaft is still a workable shaft, but did now want a shorter shaft and all the cost of changing.