Why can't they continue tuning it? The majority of the tunes I do are street tunes and they come out better than dyno tunes believe it or not. Have them do a street tune and get max timing, a/f ratio as good as they can, shift points, firmness, etc. You don't need a dyno to get it tuned 99%. What was the a/f ratio?
I don't see a big difference from 12.5 to 13.0 on a the heavily modded engines.

Just as a comparison, I tuned a trans am a while back, it had an aggressive cam 232, cutouts, headers, tuning, maf and it put out 389rwhp. That's on my dyno, a tad lower compared to the other ones.

With your mods, sounds like you should be over 420rwhp.
How does it feel on the road, besides the burn out problem? I figured you'd loose some low end, but not a whole lot.
If it's really low on hp, wonder if the cam is installed correctly. I've see them a tooth off before and they don't feel strong at all. Pushrod size? Springs?

Hang in there you'll get it!
Once you do, it'll be a very strong truck, should put you easily in the 12's.

allen