I'm pretty sure it will. The only thing is that it might sit a little lower because of the frame size difference. You can find a used rear shock on ebay for pretty cheap anyways. I would go that route.
I looked (compared side by side with 1986 T3 shock) at 85-90 LT250R shock before i sold it and it was very very close. A bout 1.5" longer, may be less travel, rezzie hose shorter, but looked to me as very close replacement
2001 raptor shock will work, just need two washers to take up some space on the top mount. Make sure to get the raptor shock with the remote res, and use the t3 hose from the shock to the res. You will need to get it rebuilt after you switch out hoses. Make sure you fit it in there the way you want it cause when you get it rebuilt you will not be able to move the hose once it is rebuilt and there is only about two ways the shock, hose and res will fit.