LB7's are cool. The mileage wouldn't really worry me. (Assuming it was maintained OK)
Like mentioned..... injectors, water pump seal, glow plugs, head gaskets are all known culprits.
The Allison trans in stock form is OK, People have had issues with higher HP tuners/programmers.
I've seen the atomic fab mounts in person, I also watched FFR build Matt's mounts on his NNBS Blk 1500. They seem to work for the lighter trucks & drag crowd. ( Limit straps can help hold the front end down too)
I would really brace that upper shock mount to the frame somehow, it was never...
Even 30 over, your 330.35ci LS based eng will not make 600 NA RWHP (on earth, assuming 14.7 atmospheric pressure) * Non- Unicorn internals
Unless you are attached to that block for some reason most would argue that it's simply not worth the machining cost for such little performance gains...
It's possible. Google it, there are plenty of rotating kits/assemblies available for purchase. Do you have a good machinist? Do you have heads lined up?
I can personally think of a few better ways to spend $3-5000+ but that's just me.
To briefly touch on engine theory, the small bore LS...
GL, If you're worried about re-ringing a cheap LS based engine or tossing in a (relatively speaking) cheap 6.0, I would heavily chew over the diesel idea.
OE style injectors for those things are like $350+/ EACH last time I checked. Do you need a diesel or just want on the bandwagon?