For my 1979 F350 4x4 w/6.6 engine, when using the factory exhaust manifolds should I have the muffler shop bend 2 1/2 " tubing from the manifolds into a 3" main pipe and continue the 3" through the muffler and out the back?

 

If thats what you want to do. I don't see anything wrong with it.

 

If that will reduce back pressure and help the engine to function better without the expense and trouble of headers and the more expensive and tricked out exhaust systems, then that seems the most sensible way to go. I want to spend what is necessary to remove the body rust permanently, repaint the truck, and I am in process advancing the cam to non-Califonia factory specs.
The use I will make of my truck makes the expense of aluminum heads, 4bbl intake, cam, and headers for just cruising down the highway and four wheeling out to hunting sites seem to be money spent that could go for gas. if you have any suggestions for where to go to buy aluminized exhaust pipes to do what I described please let me know. thanks

 

You can't flow more than your smallest restriction. Also you will loose exhaust velocity and this will cool the gases and cause undue condensation in the pipes (rust) In addition the loss of velocity will kill your scavanging effect and the added cooled gasses are heavy and will cause additional back pressure trying to push the heavier charge out. You could increase to 3" at the muffler to reduce muffler restriction. Some say reduce 1 size for the tail pipe and some say go bigger. My opinnion is to increase at the muffler and stay the same going out to maintain velocity and scavanging.
It will actually work better if you dump as soon as possible after the muffler but thats a call you have to make for what your looking for and sound control.
Of course this is for stock mani's. I use an H pipe and if you do connect as close as you can (stuff in the way) and no more than 1 size smaller.
You might get better performance by going with a 2 (2.5) into one y (3" out) with a high flow muff, 3"" into 3" out the tail. These systems work with differant physics and scavange like crazy. They can sound bad *** too. Scavanging can massivly increase air flow through the carb.
My 2 cents.

 

Your final suggestion is what I was trying to say. If I go out from the stock manifolds with 2 1/2 dia. pipe into a Y which has a 3" outlet and into a magnaflow or some such that is quiet, and continue out with 3" pipe. I would have to see how long the muffler is to direct the final exhaust. I might be able to dump it just before the rear tire [passenger side], but probably will have to go out after the tire. After I got up to a minimum of $3K but probably more like $5K for all the engine mods I was planning the result was lots of power and torque [25% increase approx] but only a 2MPG increase in milage I looked at how much gas that amount of money would buy, I was looking at about 12,000 miles worth of gas. In the last 15 years I have only put on 20,000 miles, so I just couldn't see going crazy on the engine. Now, if I was still in my forties it might make sense, but I figure I will wear out about the same time as the truck so maybe a grandson will get to do the mods.