Caddy 500 in a 62

[CanadianGMC]

Hey everyone,
I've finally got the chance to put a Caddy 500 in my 62 but I have a few questions. My truck was an I6 with a 3 on the tree manual transmission. The 500 is coming with one of 3 Turbo 400 tranny's. I have the choice of a long medium and short core, but I'm not sure which is the best length for the truck. What length would fit best? Also the main block I'm looking at is bored .030 over, the deck has been resurfaced, the mains aligned and new cam bearings installed. He also says that it has about 50km on it but it spun a rod bearing so it disassembled it, would it still be worth it? There is also a second assembled short block that I plan on picking up as well.

He says he has 4 blocks 3 tranny's and other parts and I can have it all for $500 but I don't have the space. I'm just taking a look at my options