How can I get more power and torque from my HD truck with underpowered 5.

A couple of points and a recomendation. On the 12 valve 5.9 and 8.3 there is no electronics to install a chip on, they must be physicly turned up. Simple to do just remove back cover of injection pump and loosen jam nut and adjust screw. It is within reason to boost the 5.9 to 225hp with just the adjustment, however exhaust smoke may become objectionable at near this point. Much more than this and you will look like a freight train under load. That being said I would still in this situation suggest the FL80.

Here is my reasoning, first off it is designed to handle the types of payloads you are looking at placing on this chassis.

First thing I see is brakes. The 70 is hydraulic brakes which will make it difficult to add a tag axle that has brakes on it. First off it will be hard to find a tag axle with hydraulic brakes and secondly ballancing the braking system. Being able to go is one thing but being able to whoaa is MUCH more important. The 80 having air brakes will allow it to be much more effecient at stopping, as well as in the long run brake components will be cheaper and easier to replace. The brakes on the 80 are probably Eaton standard air brake components available through many after-market suppliers. Also the 80 is better suited to placing a tag axle under and further increasing its load carring capacity, as well as, pulling a pup trailer.

Second thing I see is the axle setup, adding a tag to the 70 is not that that hard to do, but it will have to be a fixed(no liftable) non-powered axle. This will make traction poor in off dry pavement situations as only 1/2 of the weight of the truck will be on tires with drive. Also, with cost of aquizition of the axle and instalation you will(at least in my area be looking at 3-5 thousand dollars. The 80 being set up as a tandem will have drive to both rear axles. Then if you wanted to add a tag to the 80 you could get an air up/down unit.they are more readily available and easier to install.

Third is transmission the six speed while nothing really wrong with it will likely not give enough gear selection especially with an underpowered engine and then adding as much load as your talking. The 9 speed will be a much better unit.

There may also be some issue with the radiator in the 70 as working a 5.9 that hard will tend to heat them up a bit.

As far as sevice life. With proper maintenance the FL80 should see between 800-1mil km, before major repairs would be expected. The 70 should see 400-500k km.

If I were going to by the FL 70 I would just put say a 16ft, 60" side bed on and haul around 300 bushels. Not put a tag on it and use the truck as it was designed.

If I needed/wanted a larger payload I would go with the 80.

Just in my opinion I think for what you are wanting to do te 80 would be a better choice. I do like the apeal truck with lower miles, but I think its limitations outweigh the benifits of a few less miles.