BV, I haven't looked in a while and I started looking for the LS1 then figured out the difference to the power of the LS6 was mostly heads and cam [only a little cam difference IIRC], so I focused on the LS6. I hadn't found any aluminum 6.0L's at the time I was looking. All the LS1/6's were around $5G with a computer [less without when I was actively looking], for me the amount of work as you say is the same with iron or aluminum and the cost is what it is, these cars with an iron block have a HORRIBLE F/R weight bias to truly improve it you also need to move it back, but "to me" if you do the all alloy/composite with EFI and aluminum rad. etc. and deal with the 400HP of the LS6 out of the box you will only need the same "mod's" as a stout SBC that we all mostly do anyway. So if you are shooting for Turbos/blower/NOS and 600+HP you will have some spending to do. Again this is from my wish list I want a stock looking interior including the full [as it is] rear seat, this again is an advantage of less weight. I know the alloy block will not handle all of the power, the iron one will but our chassis' have some serious limitations with power, remember they were built for 4cyl. 100HP so pushing into the 500HP neighbourhood will add work and these cars are fun to drive with as little as 200HP so this is where my opinion comes from. I have little interest in "racing" I just want to drive for fun, and today, here, that means "Track Days" So for 6D9 starting this with "a budget" for a build up, I think he should stay with the 406 he has, as he has it and it is the easiest swap with as he says "a budget" as I too was already well invested in Gen I SBC so the LS6 I wanted just didn't make sense to me at that/this time [Oh I still want it, but I want to drive it too, so a smaller project has become the focus]. Pricing and availability changes day to day and area to area, I was looking eBay and my last search turned up ~10 all far away/expensive shipping while earlier searches turned up 50+ engines with a wide geographical distribution. This is just my take and it all comes down to how much he wants to/can spend and time line. 6D9 also said he is working outside if I read that right, He can always re-do it later if the right stuff and opportunity becomes available. It isn't as though I haven't changed my mind 20 or 30 times!
Harry P.S. I have original NOS 5.7 emblems and that is something that matters to me, I am surmising he has "a line" on the L33 which for most people becomes the "Executive Decision Maker"
I'm not a hoarder I'm a preservationist 78 Monza Spyder (~Soon(ish +/- I guess) To Be 2+2 with Spoilers)