I would like to hear your input about cat d4-7U with 2 speed pto pulls corn chopper or silage. Have anybody done it and wonder if it works good. 
farmerD5

Yes, many farmers did years back when fields were too wet for wheel tractors, it has been done.

Back in the 60's our neighbor used a 6U to pull a 2 row chopper. He used a 4 row mounted planter at that time. Don't remember the brand of the chopper.
That being said the planted corn populations around here were probably less than half what is planted now. Truck followed the chopper so no wagon pulled by the Cat.
Don't remember it being much different cutting rate than the similar chopper we used on a Super WD-9. Chopped corn was placed in a trench silo.

I believe the 6U was about 1954 or 55.

it would certainly drive the implement just fine standard pto speed is 540rpm.
the ground speed might be the issue.
plenty of farms in aus use tracked tractors in cotton and grain crops.

I can remember my Dad using an International T6 crawler on the farm in the 1960's to pull our John Deere 2 row corn chopper around the field as it was too wet for a wheel tractor to go it alone.

We have run combines and other pto implements on D2s and D4s. No issues other that the pto is not a live shaft so when you disengage the main clutch the pto stops. Not a huge deal but it would be nice to be able to stop the tractor and let the machine catch up when you start to plug up.

You can accomplish that by pulling both steering clutches.

8C361 - good point
Many do not realize those clutches can be used to stop all forward movement and allow one to take machine out of gear in emergency situations.
I had a pin come out of the master clutch linkage inside the housing, at the time I was discing prunes and a limb caught the radiator cap, tail seat 7UD4
No place to hide! Certainly not the time to discover a pin had fallen out!
CTS

A large farmer here had some orchard D4's with PTO's. Can you imagine having that pto shaft under your behind and so close to your ankles? They actually had operators pull brush choppers with it. I would be scared to death! I tend to think those particular versions were more suited for turning a stationary irrigation pump, not a death trap on tracks. JM