Agreed, the T5 is a decent transmission behind a mild small block, but a big block in a heavy truck sounds like a recipe for shrapnel. I ran a stock T5 for years in my low 13-high 12 second 302 Fairmont, but at 3200 pounds, thats at least 700 pounds like than a 69 F100, and a lot less torque. When I started racing my 85 5.0 Mustang in Stocker Eliminator, I thought the T5 would be adequate, as the Mustang was about 150 pounds lighter, and the Stocker engine only made about 285 HP, but within a year, I had twisted the spline on the mainshaft so badly, that I had to use a hammer to bang the slip yoke to remove the driveshaft, cracked the main case, and knocked several teeth off 3rd gear. After that first year, I had 3 broken T5s to show for my efforts.