My 92 W 250 4X4 Cummins 5 speed has a GVW of 8510 and is the same body style as the 89 (thats by the Door sticker)
I really don't see a problem if you stay under the axle and tire rating as it rides like a JackHammer with the almost Cast Iron springs.
My 96 with the same options has a GVW of 8800 Not 8600. I am close to that with my Apache 865 with AC etc. These are Both Regular Cab.
These Old Trucks are a lot lighter than you think as most are standard cab and not the busses you see now. My 96 with me in it and tank full scaled at 6520 and the box empty. Never weighed the 92.