I would get a new stock TB. Start there, may be restricting or something.
I hit 90 in my truck with a 15mph head wind, 33's and a blown plenum. You can do better. lol

 

8,000lbs of truck? I weighed my 97 4x4 Sport with a 360 atx with my sisters barrowed fiberglass topper from her truck, I had My sister,her baby and my 10 year old nephew with me on the scale when we took 200lbs of aluminum cans into the junk yard 4 days ago and with all of us in my truck with the 200 lbs of crushed cans we tipped the scale at 5820lbs!

Second of all, my parents 1992 DMC decked out fully customised Dodge Van with 4 captains chairs and full rear bench seat weighs 6900lbs with nobody in the van and will do 105mph before the sloppy steering scares the living fk out of me and I don't even floor it to get there! That van has a ton of power and even more top end power, very impressive for a 318 Magnum with 332,000 miles on it(never rebuilt! & never had a plenum gasket change!), it could definitely do more if the steering was tightened up and I was on a straight highway.

Now with that said I haven't broke 85mph with my truck but it certainly wasn't because it couldn't do it, its because my tires aren't the greatest and thats where I stopped pushing it, didn't even have to try to get up to 85mph it was there already after blowing by a dick who wouldn't let me get in the right lane to get off the highway.