You have to make an honest attempt at an apples to apples comparison, so start with a /6 car...
Price the following:
running serviceable 5.7/trans assembly with all accessories and electronics
Unless you have the time and patience to do the wiring yourself, consider how much it will cost to modify the factory harness to run in your vehicle
Include appropriate driveshaft modifications for your given choice because I somehow doubt any of the factory lengths/splines are correctly suited to just drop in a 3G/trans combo and go, and even if it did, unless you just had it rebuilt, the factory u joints are probably not in the best shape
Chassis and sheet metal mods--be honest here, unless you plan to use a new hemi and slap a TF behind it, you're going to modify the floors somehow, even if you're a competent fabricator, you're paying for materials and time--consider some people have to pay for this because they don't have the tools/time/talent. This also includes any T-bar X member mods, engine mounts, etc
Include any upgrades you plan to make during the swap: Cooling system, headers/exhaust, suspension/steering, induction, ignition, etc--because most of us aren't happy just slapping **** together as issued, nor do most of us like to reuse parts unless they were recently installed/modified
Take that number and add $500 for piddly **** like gaskets, sealants, petroleum/oil/lubricants, coolant, the stuff you broke/forgot about/needed during the swap.
Now do the same for a 5.3/4L60 swap taking into consideration many of the swap parts are WIDELY available and your Google-foo need not be strong to find what you need, grasshoppa.
If you're starting from ground zero, like many of these folks are, the LS will be the victor--despite the minor fab and parts swapping involved.
Then, as a third option, consider a 360/904 or 727 or even the OD trans swap...how much would you spend for a fully serviceable engine/trans that equaled the power of either option above...after you take a long hard look at the money and work involved, while it may not be your cup of tea, it's hard to argue the swap with those who just want miles and smiles for not as much money and are willing to sacrifice some brand loyalty.
I've had to reconcile for a long time, being a Mustang fan, some people want a cheap light platform and an easily modified engine that responds very well to aftemarket mods...can't tell you how much SBC/BBC/LS swapped Foxes I've seen.
