Since I'm going from a 346 to a 408 I know I won't be able to drive the car untuned. Now my question is will it be ok to take the fresh engine and throw it straight on the dyno with no real break in?

 

I'm doing the same thing but I'm getting a mail order tune for my break in period. Would suck to ruin a new motor. Not saying it will but I'm guessing you went with bigger injectors (I did) and you don't want to wash out the rings. This is just my opinion and how I would do it. Good luck.

 

Why can't you? I broke my engine in with the stock LS1 injectors when I went from 346ci to 376ci. Just keep it out of WOT and I think it will be okay. As long as the ECU stays in "closed loop", it will maintain safe timing and A/F ratios.

 

Get it on the dyno, have the tuner put together a quick base tune to start it up and check for any leaks etc..

Once up to operating temps he can do 5-10 min's worth of 3-4k rpm pulls and let the engine do its own braking back down to idle on each pull, that will create vacuum and clean any debris off the rings and blow it out the exhaust..

If the car is a auto car then make sure the vehicle is in manual 2nd or 3rd during the pulls since you want engine braking during these pulls...

After 5-10min of that it will be ready for WOT pulls!

 

When I did my 370 I pulled the ecm and had the tuner burn a base tune, so I could drive it to the dyno for the real tuning

 

^^^^^^^^^^^
What he said.
When on a dyno the tuner can put load on the engine making it work & bed in the rings.

 

I'll be running the same 42s from the ls1 but I was told not to drive it on the ls1 tune. That was originally the plan.

 

Truck it to the tuner! let him retune it on the dyno & run it in at the same time.
Why would you even want to drive the car with a tune for a completely different engine.

 

Most piston ring manufacturers will have a break in procedure. Like running the motor at double idle speed until normal running temps to seat the rings properly.

 

Running at idle or double idle doesn't create enough cylinder pressure to push the rings out Into the walls to seat them...

These engines need to see WOT scenarios quickly so the rings don't waste any time seating.. This is especially true with engines running steel top ring sets which need to be beat on hard to seat them...

You don't want to create a weak seal

 

I didn't make it up. It was listed in the directions that came with my rings. Please note these are stock replacements and not aftermarket rings I'm talking about.

Not that's not to say I disagree with you. My new Jeep had nearly the same directions as yours for the first 50 miles or so.