Here's what I was able to find on the web.
On block strength. First off let me say that the stock block 5.7L / 6.1L Hemi is ridiculously strong. I can't think of another stock block motor that can withstand 1000 HP without major modifications or outright replacement with an aftermarket block. That being said, the consensus is that both blocks can survive 1000 HP in both a stroked AND boosted applications. It is only in VERY high horsepower applications were the 5.7 block has some benefits due primarily to its thicker cylinder walls.
It appears that if you are running BOTH a stroked and boosted combination the 5.7 with its thicker cylinder walls might have an advantage. I've found a couple of postings where people were running 6.1L Hemis bored and stroked to 7.0L and running over 20 lbs. of boost had failures in the block. Keep in mind these engines were well in excess of 1000 HP. Thus, if a running a boosted motor with over 20 lbs. of engine boost it might be a good idea to find a suitable 5.7L block.
What I found most interesting is that although the consensus is that 5.7L Hemis have thicker cylinder walls and is a stronger block, I was only able to find one set of actual cylinder block thickness measurements and those were for a 5.7 with an average cylinder wall thickness of .170", which is not particularly thick and for me I would not feel comfortable boring such a block .080".
As far as overbore, the Chrysler factory service manual claims that both blocks are not to be bored, and instead replaced when cylinders become out of round, or tapered. That being said, it is clear that nearly every 5.7L and most 6.1L blocks can handle at least a .020" overbore. Personally, I'd sonic check any 6.1L prior to boring as they appear to have thinner cylinder walls, and if you are unlucky enough to have one with excessive core shift you might have longevity issues even with a minimal .020" bore increase. .040" pistons are commonly available though sonic checking the block would be required.
Although I've read claims of a few 5.7L blocks being safely bored to the stock 6.1L 4.055 size, I'd consider those blocks freakish. Thus, if maximum displacement is your goal a 6.1 block is the place to start, unless you are going to sleeve it.
Conclusions: In my OPINION, if I were going for a maximum output from a naturally aspirated engine with the largest possible displacement. I'd opt for a 6.1L block as my starting point. If I were going for maximum output from a boosted application with a STOCK stroke I'd still go with a 6.1 block. The stock stroke with its long rod has far less side loading on the cylinder walls.
If I were going for maximum output from a boosted or nitrous motor with a 4.050" or 4.080" stroke I'd probably opt for a hand picked sonic tested 5.7 block with minimal overbore. This would be particularly true if I was in excess of 1000 HP.
Finally, if you are willing to use block fill, I don't think it would matter at all which block you chose.
Regards,
Joe Dokes
