I had a 69 4520 that I added the intercooler to and changed to a high pressure pump with new injectors.  It was at about 165 hp and handled great.  It was an overgrown 4020 that had a better ride and much better handling than similar size tractors for it's day.

Many people didn't like them because of not enough power but as was mentioned they were a heck of a tractor when juiced up a little.