I personally love the 7.3 in my 2000. I have about 200K on it and the worst problems I have are from putting a 75/100 chip on the truck! It is a DOG otherwise. I resolved those issues by dropping to a 50/75 chip. I have blown the turbo piping apart, hand normal alt. and battery issues, but other than those, the truck has been super reliable. (Maintenance is a big MUST!! Change the oil and antifreeze regularly!!)
I had a friend with a 2003? with the 6.0. It was faster than my truck, even before he put a tune in it. His truck was even better, after he put a tune on it! He could barely get 13mpg out of his truck towing and I was usually good for about 17.2, depending on conditions. His truck also sucked unloaded, but he also loved blowing black smoke all over the imports. He popped his head gasket, just like clock work. Right at 80k, he had to fix it with the head bolts and gaskets. Not really that big of a deal, as we knew it was coming from what we had read.

The only differences I can think of with the Lariat vs. the XLT was the stereo and leather. I used to know it, but I have forgotten. The leather only comes stock in the Lariat or just order the seat covers yourself.

I might be wrong on this, but prior to 2000, the Super Duty truck had an Allison transmission in them. In my opinion, those are better transmissions than the 2000 and up. I would have preferred to have that transmission, but the 4R100 hasn't been that bad. I upgraded some parts and it has been a solid performer. I am not a fan of the way it shifts, but after 200K.... who am I to complain!

I really wouldn't want to deal with swapping the motor into a 150 model. I really doubt the suspension would hold it, without major changes. I mean, if you have the skill, time, and money to waste.... I would just find the right deal out there. Dealers aren't moving these like a few years back. They have to be out there for a decent price. And if you are just going to be pulling an open trailer with a tractor or two, there is a guy in my area that has been that with a old 70s in line 6. It ain't fast, but he has to have saved a ton on his rig!