So I've got the 5.3 mated to the T5 with a 621 bellhousing.
Everything is seemingly as it should be, except for the starter.
I've done some research and have found that with the 621 bellhousing, an old iron nose cone type starter is required owing to the clearance inside the bell.
I have also found that a mini starter without a nose cone will work.
However I'm on a budget, I'm going to attempt to clearance my (PG?) - (newer) type starter from the 5.3 donor truck. I'm under the impression that with shouldered starter bolts, regardless of what I clearance on the aluminium nose cone the starter will properly align.
I'm writing this as preliminary notice. I'll update this later one.

Don't over complicate LS starters, they're very simplistic

Nose cones fall into three categories:
GTO which had a unique BH due to the RHD packaging since it's actually australian
LS4 FWD trans mounted
Every other LS ever made

Pad/bolt refers to the earlier short/long bolts that attach it to the block. The early short/long can be replaced with later long/long by simple replacing one bolt (Dorman sells then cheap)
It makes no difference to the BH or trans

If your LS truck starter touches the 621 BH, then whip out the die grinder and make room. Not much to it

621 bell with a T5? Interesting..

After paint marking all the areas of contact interference, both before and after removing some areas on the nose cone. I just figured Id suck it up and just went with an adjustable Amazon mini starter. which fits well. I would've been wise to check for interference before the drive train was in the car.

Amazing sometimes how many things are like that.

Glad you got it to work, in any case. Lots of the "mini starters" will work in all sorts of troublesome situations, since they're ... smaller. Lots of OEM starters will too, with a little bit of wizz wheel or die grinder effort. It's one of the more readily conquerable problems one might run into when doing this sort of thing.