I tow with an '84 D100, RC, SB, 225, 833od. I de-smogged the motor, installed a Super Six 2bbl and a 2.25 exhaust without catyl convts.
Trailer weighs 1900lb, car 3000lb, takes a bit to get to speed but otherwise tows fine. I have towed a 5000lb JD tractor a short distance (10 miles). Trailer has 4 wheel brakes, truck uses 5000lb receiver hitch without load distribution bars. I adjust the load so I have ~10% tongue weight.
With the original 8.25 rear and 3.21 gear, it was hard to get it rolling. Now with a 8.75 rear and 3.91SG, taking off is not a problem, unless it's a very steep incline.