As someone mentioned above, I'd hate to see you go down the Mopar or even Cadillac path, only to get in over your head and be forced to sell the unfinished project at a big loss.
One of the engines available in an 84 pickup was the 454. Probably just the C20 and C30, but still it means just about anything you need to install a big block will be standard off-the-shelf stuff, AND WILL FIT with no mods to the frame, firewall, etc. And there's gotta be 5-10 times as many aftermarket parts for it compared to a Mopar or Caddy engine.
You could have a competent trans shop go through your TH350, assuming you don't plan to run slicks and smoke the tires at every opportunity
. Or just jump right in and get a TH400 or a built 700R4. I don't think the BBC-equipped trucks (or the C20 trucks, for that matter) used the C10's 8.5" 10-bolt, but those things can live behind quite a bit of torque. However, my guess is aftermarket axles would be in order.
A drivetrain note: The big blocks are so torquey that you could get by with a 2.73-3.08 axle and still smoke the tires. That would mean you wouldn't need an overdrive trans.