5.3 6.0 8.1??? To many options

[Thuddy]

Ok guys Have a 77 K20 currently has a 400 SBC and a 700R4 Wanting to do an engine swap now. Have been contemplating doing either a 6.0 or an 8.1. When all of a sudden I happen on a complete 2000 Chevy has been wrecked, only it has a 5.3 in it, also has 160k miles on it. But would have everything i would need for the swap. Keep in mind my dad drove this truck off the lot in 77 so #1 I want it to be done right and fun to drive when finished. I can trade one of my AR 15's even for this truck. So not a huge expense on my part. Would you keep looking? Would you do a larger displacement LS? Wouldn't mind being able to put the horse trailer on it everynow and then when we ride close to the house, but no long hauls that's what the Duramax is for. I know this is a long and involved post but maybe you guys can help me out.