Turbo sizing is VERY complicated. No system does everything the best - so it all depends on what you want. A turbocharger is a supercharger driven by exhaust gas instead of a gear-drive or belt. So, a turbocharger-unit has an impeller, a compressor, and an adaptor between the two. Varying any one of those components changes performace at a certain range. There's tons of info posted on the Net that you could read forever. Often, a non-turbo engine will run better at low RPMs - partly because it has a higher compression ratio.
In regard to inline engines being heavier built than V designs. A V engine can also be built heavy - but they don't exist - that I know of - for light trucks. I've taken connecting rods from 6.2/6.5 GMs, 6.9/7.3 Ford IHs, Duramax (Isuszu) GMs, etc. and compared to the rods in the 5.9 Cummins. The difference is amazing - the Cummins is MUCH heavier. So is the block and so is the crank.
That being said - I've got a 6.2 that lasted over 500,000 miles - but I suspect it was a freak. The crankshaft finally broke into three pieces. The 6.5 is basically a 6.2 with slightly larger bore and a thinner block casting.
You asked about boost levels and the 6.2/6.5 blocks. First -the blocks vary quite a bit. The early 6.2s were heavy and well built. The first year 6.5 was also heavy (1993). But, in 94 it was cheapened and made thinner and lighter. Also, the alloy was changed early on. The first 6.2s in 1982 were high-nickel content blocks - but later on the alloy was weakened and blocks are more prone to cracking and main bearing webs pulling out. Now, International Engines (part of Navistar) is casting the new blocks for the 6.5s and they are supposed to be the best ever - high nickel content and thick castings - and also very expensive.
You've also got cooling issues to contend with. If you make more power - via turbo boost and increased fuel delivery - more heat is made. The later 6.5s have cooling upgrades that work better than in the early 6.2s.
And, you've got the compression ratio to deal with. Turbocharging raises the "effective compression ratio" because it forces more air into the combustion chamber. Intercooling does the same since it condenses the air by lowering the temperature. So, if you take a non-turbo engine - and put a turbo on it - your options for boost are limited - usually 10 PSI is a safe level. Take a 6.5. It came as turbo version or naturally aspirated. The NA has higher compression pistons. Several companies used to make special low-compression pistons for 6.2s so they could be run at high-boost - but I don't see them available anymore. The various 6.5 pistons are still available. Also, the turbo 6.5s have pistons that are not only have lower (mechanical) compression ratio, but also ceramic coated skirts to better handle high horsepower levels - and some later 6.5s also have oil-squirters in the block to help cool and lubricate piston skirts.
One other issue is that of propane injection. It does next to nothing with a non-turbo engine - but with a high lever of boost - can give incredible gains.
That all being said - if I wanted a super-high boost engine - I would not pick any of the 6.2/6.5 GM series or the later Isuzu Duramax. The OEM blocks and cranks are not "overbuilt" whereas, something like the Cummins is. I suspect though, that the new 6.5 blocks being made can handle a lot more abuse then the older OEM blocks can.
Hell's Yeah!! Now thats what I'm talkin' about!! 
