Reconnecting 50 amp service to outbuilding
You'll need a pull box and some insulated mechanical ("Polaris") connectors for this

You are correct that wire nuts and an ordinary junction box won't cut it here.  Instead, what you need is a correctly sized(6" by 6" minimum unless you have fatter conduit than 1", which will require you to go up to 9" by 9") pull box rated for the environment (NEMA 1 for inside, NEMA 3R for outside) to house the splices, and then insulated mechanical connectors (Polaris/Unitap/...) rated for the gauge of wire in question to make the splices.  You'll also need to use a torque wrench to make up the connections -- we're talking torques on the order of 10-20 ft-lbs here.  Last but not least, you cannot bury the box -- it must remain accessible so that the next bloke who needs to mess with those splices can go in and do so without having to demo part of the building on a junction-box hunt.