BigHog, please listen to buoymaker. He made excellent points succinctly. I'm going to add some grim detail from personal experience. If you get the right T5 and rear end ratio, you can make your 235 powered pickup go as fast as you want and a whole lot faster than is safe. With the setup I have, at 80 mph my 235 will be running about 2800 rpm. At 85 I still wouldn't be at 3000 rpm yet. But having owned these trucks for 50+ years I can tell you even traveling in a straight line and going 75~80 mph in an AD truck can put you, everybody in the truck and other drivers on the road in grave danger. Absolutely nothing at all about the stock AD trucks was designed for 75~80 mph driving. No matter what anybody tells you.
In the 60s, 70s and early to mid 80s when scads of these trucks were still being driven around Texas (before the days of T5 transmissions) I came upon a handful of serious accidents involving what were basically stock AD trucks into which small block v8s/automatic transmissions had been installed...with the sole purpose of making them go faster. There was even one man in South Dallas who advertised he'd turn your 1950s Chevy pickup into a "V8 powered freeway monster" for $1100. And he would, but he always left the front axle alone. What he was doing was about as irresponsible as putting that SBC engine into a 3 wheeled golf cart. After doing this about 3 years, the guy disappeared.
Two of the accidents (one in 1978 and one around 1981) were single-vehicle wrecks where the drivers had simply been going too fast on a straight stretch of highway and something happened so that control was lost. Both were fatalities and I doubt the driver in either accident lived more than 15 seconds after the truck started flipping. One had so much blood inside it looked like a human had exploded in there. The other had caught fire from the gas tank which of course sits right behind you. One had flipped to the left and had stopped on the other side of a freeway. Fortunately no other car was hit when it happened. The trucks both looked like they had been forced through some bizarre garbage disposer and I have no idea how many times they rolled over before stopping. Very little about them could be salvaged.
You have a solid front axle with a very tiny caster angle (2~2.5 degrees or so) that was designed for a maximum speed of around 60 or 65 if everything is all Jake in your entire front end. That's not to say you can't drive 70...you can, but at that speed you're flirting with the outer limits of safety from many different standpoints. I know...people here will show up and say "oh baloney...I drive 70 all the time." My suggestion? Take your pickup out and drive at 70 with a stock front end. See how precise and accurate it feels to you and then let's talk about it. This tiny caster angle and unforgiving suspension means at speed your pickup can become VERY unstable given the slightest variation in roadway, a rock, a gust of wind, a shake of the steering wheel, roadkill, some driver distraction...nearly anything. And only God is going to let you regain control at high speed in an AD truck. It is one step better than the old WWII Jeep. And yes...I know...Jeeps today have solid front axles and they all drive at freeway speeds. They all also have power steering, a caster angle between 5 and 7 degrees and camber and other front end settings specifically set to keep the Jeep as stable as possible. If you put 5 to 7 degrees of caster in your stock AD front end you better be a HugeHog. With that much caster you won't be able to drive it anywhere unless you have arms like Bluto...but it will not wander or wobble around on the road at higher speeds.
If you want to drive at the speed some people today are driving pickup trucks, please go ahead and buy a new or newish one. Something made since around 2008 or so, but do your homework. See how professional drivers have rated the handling characteristics of various makes/models. Check the IIHS, also.
