Alright, so I have picked up some different combinations and now I am not sure what to do with them.
I have numerous motors, three of which are high mileage LS based Vortecs.
What I have now is:
2004 lQ4 (I think, that's VIN "U", right?) with ~430k and a knock, complete with all wiring, computer and 2WD 4L80E, no accessories on the front, though. Rebuild kit looks like ~1200 + cam.

2004 4.8L complete still in van with ~590K runs smooth and drives but only has about 5 PSI on the oil gauge. Probably could get by with basic rebuild (pump, lifters, bearings, rings)?

2004 4.8L complete still in the van with ~372K runs smooth but when driven felt gutless. I was told the transmission is going out, but it acts more like it's dragging the brakes or super over loaded. Don't know what it needs yet, just got them this weekend.

BTW - all original motors for what it's worth - these things last forever!

I can also purchase from the local recycler a lower mileage (40K-120K) 5.3 for anywhere from $250-$500 but that doesn't include intake, wiring, computer or accessories. He also regularly gets 6.0L Vortec engines in with the same deal but a price closer to $700-$1000, depending on mileage.

ANOTHER recycler can get me all accessories (no wiring, intake, or computer, though) for less than $200.

Additionally, I have someone telling me he will sell me his complete ~180K mile 1991 Cummins Dodge 2WD this spring for $1500.

I also have several BBF (460) engines sitting around waiting to be rebuilt. I can rebuild one of these and make ~365 HP and great torque for ~$1000, but it will never get better than 10 MPG.

Anyway, I also have a 1993 K3500 (CCLB) I wanted to do an LS architecture engine swap into, and this is about a 6K lb truck. I am curious as to what everyone thinks that I ought to do for a motor. I HATE the old SBC motors and mistakenly rebuilt the TBI 350 once only to have it fail within the first few months. I had little money at the time, which was part of my issue, but it still should not have gone THAT fast considering I spent ~$1,000 on it. (My father still rubs in that he told me to buy a crate motor.)
So I am done with old SBC motors EVER and am going to swap SOME motor into my tonner. I have a divorced 205 to go in, so 2WD transmissions are okay. Mileage is important but for the price of some things I can buy a lot of fuel.
Anyway, for the money what do you all think is my best option? I want to keep this under $2,500 if possible and still have the best combo possible.

Add to this that the vans I have (two) are ALMOST too nice to part out. They are not rusty and have only minor cosmetic issues. They are 3/4 ton cargo vans, which in my experience ALWAYS sell, no matter how bad.
On the other hand, I am not sure how much money I want to put into a couple of high mileage vans and HOPE to get my money back.

Thoughts?