I built one but didn't use the rear end of the tractor. I used a z turn pump/control unit along with hydrualic motors. I posted the full build in an earlier thread. Let me know if you have more questions.
I used 100 pitch roller chain for the tracks and welded the plates right to the links. I do wonder if something like double pitch chain might be a good way to go. This would require fewer plates to be welded. Also somethink like a conveyer chain (https://www.surpluscenter.com/item.asp?item=1-2652-2060&catname=powerTrans ). Basically I started with sprockets and based that choice on finding a good price and happen to find 100 pitch sprockets which really drove the chain choice. I do wonder about using a two chain system similar to Struck (Struck Corp; Magnatrac? Compact Crawler Tractors ) which might allow the use of a smaller pitch sprocket (two on each end) along with the double pitch or conveyer chain.
A while ago I found this web site (Mini Bulldozer Plans ) which sells plans for the conversion.