It really does suck you can't use the 05 and up 3.7; but with money comes magic. unfortunately, the money you would need to convert is more than getting your year motor, or just having yours rebuilt. I got lucky with using a 1994 ford explorer 4x4 motor in my 1993 4.0 Aerostar (gave it to my brother, then he sold it, how nice), lots of the things where different, but they made it work.
Just as an example, On the 1994 motor:
No crank position sensor on the Main pulley, but the MOUNT hole was there, so they had to use my crank pulley and mount the sensor there. Intake was completely different, and had to be swapped from my 4.0. The intake manifold on the 1994 had an extra vacuum hose nozzle, the idiots didn't plug up correctly, it fell off and ran like crap, but I found it ad put a Jell cap on it. Some other problems that they had, but they got it to work.
Then they SAID They could not get the 1994 motor to bolt up to the transmission while the trans was IN the thing, so they had to take that out, and guide it all in from the front. In the process, they broke all kinds of shit in the engine compartment, so I ended up not paying for $150.00 of the install cost. They wanted 600.00 to install it, I paid for HALF up front, then he wanted a laptop for his kid. When they were done, he decided he didn't want the lap-top. All I had was 150.00 on me, so he took that to let me take the Van, and I owed 150.00. Well, it took them hill-billies SIX MONTHS to install the motor, that had a 6-month warranty. THAT ran out before they even installed it... YES, it was an Auto garage, not a back-yard shop, they just fussed around with mine. They had no idea what they were in for, and said they would have charged me more if they knew what they had to do to get this other motor to work. But after breaking the Driver headlamp socket, MAF sensor, Air Box, and all the wiring harness fastener holes, and what not, I refused to pay the last 150.00 unless they repaired what they broke. Since that would have cost more for THEM, they had no choice. Then, summer rolls around, to find the A/C doesn't work, and id DID before they replaced the engine. So, I figured they did not rechange the system, so I did, and all the shit blew out the back of the compressor where the hose fittings are. They did not put the "O" rings back in either. They made a total mess of it, but it ran... The plug wires laid all over the place, and looked like spagetti in the engine compartment. But, that's what I get for using Gomer, Andy, and his pals...
I also provided platinum Plugs, plug wires, and synthetic oil & a filter, for which they didn't use, I found out MUCH later. They were supposed to replace the valve cover gaskets, they did not, and it leaked oil a month later, and there was nothing I could do to EASY fix that, so I let it go... THEN to top it all off, the guy who took over the place, stole the Payphone when he closed down. So, as friends and business professionals? THEY SUCKED!
The umm, silver lining? Since the 4.0 was from a 1994 Explorer 4x4, a heavier vehicle; I had the higher rated HP/torque motor due to the cam difference, and I could launch this van like no body's business. The problem with that was, it fell flat on its face at about 70, but from 0-60, you weren't beating me, it was awesome. I could drag this van from light to light and surprise a lot of people. I drove it with the 4.0 it originally had; and this other 4.0 was a HUGE difference in low end power, liked it, but it had the disadvantages, like 12 MPG, but it was by far, the best 0-60 car I ever had, even this Jeep can't get there before my Van did...
and thought it was going to go away 
