I have a 90 3.9 (pre magnum?) in my 48 Plymouth coupe that is using the 90 OBD1 throttle body intake and EFI.

That 3.9 has some "health" issues", a Magnum 3.9 (92 & newer) is pretty easy to find.

I would like to keep the OBD1 because its there and functions, and I really don't want to have to rewire the car. Can I swap the beer barrel intake from the 92 motor with the 90 throttle body intake? What else may cause a problem?

While I'm asking dumb questions, is there an internal difference between a manual trans 3.9 and an auto trans 3.9, like a different center thrush main bearing?

You can re-drill the magnum head to accept the LA style intake. I just made a jig to drill the V8 magnum head for any LA style intake. As long as the port layout is the same on both heads. Magnums don’t have a heat crossover to heat up the intake is the only downside. Not sure about the balance if any between the two engines being a V6

Rather then re-drill the head, don't I have to change the angle of the bolt holes in the intake? Or am I remembering something else?

The car is mostly a summer cruiser, so the heated intake shouldn't be much of an issue.

Yes when drilling to LA type bolt pattern

I have swapped intake using both methods, drill the heads or drill the intake, drilling the intake is easier but turns out a lot uglier.

Also I don't remember if the V6 does this or not but the 360 TBI has the intake heated by coolant as well as exhaust. Either way I wouldn't worry about it as it would be fine even with no heat in the intake and it probably isn't driven in extreme cold either I'm guessing.

Please correct me if I'm wrong, but I thought the issue was the angle the intake bolts pass through the intake and into the heads are different between the two motors, but the locations are the same? If I change the angle the bolts pass through the intake, do I also have to adjust the angle the bolt head seats against the intake, or is it minor enough difference not to cause an intake sealing problem?

I would be installing the LA intake on the magnum heads. I would assume that re-drilling the heads would require tapping the holes as well? That doesn't sound like much fun to me, given I only have the basic hand held drill & taps. If I can get by with hogging out the holes on the intake with a deburring cutter on a die grinder to make it all work, I don't have a problem with that. I'm not concerned about what it looks like, only that it functions. This is pretty much a closed hood ride, no one wants to see a V6 in an old car.

Is there a link, or U tube covering this procedure? My search abilities usually don't turn up anything helpful. The magnum motor sure sounds like a better option if the intake bolts are the biggest problem, and that is something I can do at home. There would be very few machine shops around here that would do something like that without requiring the heads and intake being brought into them.
Pulling the heads would be a deal breaker for me.

Thanks for the help guys! Gene

Here’s some pics of my jig

Another

New bolt

As you can see now either intake can be used.

If you don't want to pull the heads then you can modify the intake itself, first grind it flat for the bolt head to seat flat and so you can drill the holes vertical in a drill press or something, then mark them and drill them, go slowly because the drill bit will be wanting to jump around and all sorts of stuff as you go thru the original bolt hole. When you put the bolts in put some sealer on the threads and it will be ok. It does not look pretty when done but it can be done. I suppose if you wanted to fill the original bolt holes in the manifold with JB weld and sand them all down you could make it look pretty but I am not good at making stuff look good, just run good on a shoestring budget.

If I remember correctly, one of the mopar magazine did a LA intake on a magnum. They use thick wall tube and made a short spacer using a chop saw. The bottom side was cut to the intake face angle (45?) and the top was horizontal. They then opened up the intake hole. This way the vertical bolt went thru the the spacer to clamp the intake. Simple, dirty, an apparently it worked for them.

Any updates or progress made?

Nothing new at this point with me. This will probably be a late winter or an early spring adventure, March or so. The car is parked for the winter.

I'm also contemplating taking another shot at replacing the lip seal, with the thought this old man may have just screwed up the last application. That sure sounds easier then a motor swap at this point.

Understood. It will be interesting to see the end results of everything. Keep us updated and thank you again.

Will do.

Any updates?

At this point the entire project is on hold. The car is for sale. If it doesn't sell, it will probably end up in storage and become someone's barn find in 20 years, after I die. Or maybe in a few years I will revisit it with a fresh outlook and then the whole thing may be updated.

Thank you for all your input, your insight is appreciated.

Thank you for the update. It was worth a shot.