I believe you are thinking of the 7.3 as the "throwaway" motor. The 7.3 is a 6.9 that has been bored out 110 thousandths (.11"). Because of this the 7.3 has cylinder walls that are so thin that you really can't bore them out anymore without ending up with cavitation holes in the cylinders. About the only way to rebuild the 7.3 is to have the block bored and sleeved - and even that is a crap shoot since if it isn't done perfectly the sleeves will drop and crack the block.

The 6.9 on the other hand has nice thick cylinder walls (nearly 1/4" of an inch) and can be bored 10, 20, or even 30 thousandths safely - and still have cylinder walls that are much thicker than the 7.3 had when they were brand new. If you are just rebuilding to spec or even to add a moderately boosting turbo - 12 PSI or less - then you don't even need to do much of anything special. Just have the cylinders bored enough to get 'em straight & true, and get OS pistons to match the overbore. Have the crank checked and maybe polished, new OS bearings as needed, and new head bolts, and rework the heads - then put it back together.

If you want to get nuts and go for boost numbers in the 15-20 PSI range (like our buddy Dave Sponaugle), then add ARP head studs, a turbo IP and a set of turbo injectors to the parts list. Maybe add a set of 7.3 rockers to get you a little better valve train.

The 6.9 is definitely not a "throwaway" - a lot of people who have had a 7.3 take a dump on them (cavitation for example) have started with a 6.9 and built a nice motor.