Death is a Knockin

[cebra]

The truck is my daily driver, I have put about 15K miles on the swap and I got a knock developing. It is a 4.8L I got from a junkyard for $900. I will continue to drive until it blows up but I would like to upgrade to a 5.3L, I was looking at the 385hp 5.3L upgraded motor for $3.2K and just a stock 5.3L for $2.2K from Autozone (Summit is similar price). I am curious whether I can turn my 4.8L in as a core on the 5.3L...the 385hp crate motor doesn't have a core. I don't hot dog my truck at all but I am concerned that my NV3500 trans may not be able to handle the 385hp for long. Opinions on what to go with?