OK, I was wondering if it could be done......and it can!!!
My 97 4.2 finally gave out at 140,000.......I priced used 4.2's and could not believe how much they were. The cheapest one I could find was a 80,000 miler for $1000, the next cheapest one was a 70,000 miler for $1400
A friend suggested why not use a late model 3.8 out of a Mustang.....I did some research and found out that the long blocks were the same. I bought a 2000 model 3.8 with 5,000 miles on it for $600. Now it was a job to change everything over, oil pan, engine mounts, 4.2 intake and exhaust manifolds had to be taken off my 4.2 and put on the 3.8, but for the price it was worth it.
I now have it all back together and running. I have put about 1500 miles on it since the changeover and honestly I can't tell a bit of difference in power or performance. In fact it feels a little peppier than before.
I would strongly recommend this change for anyone having trouble with their 4.2

 

Thanks for the info. That's good to know. I'm surprised the 4.2 is so much more $ used than a good 3.8.

 

Wow Barry, that's awesome that you took a risk with that motor. Do you notice any difference in fuel economy? Was there anything that didn't work correctly in the conversion? Pilot bearings/fly wheels or flex plates not fitting? (not sure if you have automatic or 5-speed). Thanks for the update.

T-bone

 

My truck is a 5 speed, I had to use my old flywheel on the new setup because the Mustang 3.8 manual flywheel was weighted and my old one was not. But other than that, everything just almost fell into place. Fuel economy is about the same. Drives like a new one now....I am HAPPY!!!!!!

 

There is not much risk as the 4.2 is a stroked 3.8. You probably can't tell much difference because this new motor is fairily young will the old one was worn and weak. Our split port intake manifold was used to raise low end torque so installing it back on helped you out.

 

Wow! Barry!
More DETAILS! Did the A.C. Unit, alternator, power steering fit?

What kinda motor mounts did you use? Did you have to modify?

You got any pics to post?

 

Everything fit perfect. Just bolted right on. Used the truck mounts.
The 4.2 is exactly the same engine as a 3.8, the only difference is the stroke. I am a Dodge enthusiast when it comes to muscle cars so this is the best example I can use.....here goes......
In the Chrysler small block engine lineup, the 273,318,340 and 360 are all basically the same long block. Only different valve sizes, different stoke and different cylinder bore sizes make the difference in cubic inches. The all look the same because the all use the same long block. The 3.8 and 4.2 are the same way, the blocks are the same, just different stroke. The 4.2 is a stroked a little different to give it more torque for the truck applications. But I really can't tell a difference in power. Sorry, I don't have any pictures of the engine changeover