There are plenty of photos on the Site showing how the folks here built their M53 semi's. Here's a couple.
Basically, repair your forward shroud bushing, making certain that the barrel has a few thou" clearance between the front bushing and the rear stop.
Welding being what it is, the common "EZ-track" jigs aren't always successful.
Just purchase the best (hardest, straightest) 5/16" NF X 48" threaded rod you can get. This goes through the barrel and out the back, all the way through.
Welding the metal sections is not for anyone in hurry. As you weld, adjust your beads side-to-side, more or less heat, so that the threaded rod continues to come straight out the back.
If it's crooked, back up....You don't want to build a banana.....
As it cools, it will crawl all over the place. Fix it. Experience will help. That's why this is a "hobby".
Best of luck.....Phil