In all honesty I can’t seem to source one locally in the Detroit area tried all the wrecking yards and pull a parts.. the gen 6 complete is only $550 with trans ecm and harness
Yeah, sometimes as with anything it's a cost benefit question.
Not sure on your plans for the drivetrain you do grab, you may come out ahead buying a good running but rusty donor in your area over a junkyard unit - whether it's 454 or 8100. Most of the motors have been worked hard and wore out before hitting a junkyard. It used to be for a squarebody 454 in the 120-160k mile territory you were often near the end of life. By the gen 6, well, plenty still failed at 100k and plenty lived to 250k. It really depended on how it was used, and, how it was maintained. Religious maintenance and never worked hard could be a 300k+ motor but not much of the time.
What drew me to the 8100 was all the testing GM did in development to use it as the 8.1l captains choice in the mercury marine applications. It passes what they called the marine dock test- 300 hrs, basically non stop, WOT for marine and 1000 hrs basically WOT non stop for truck. They take a 300 hr window without a break, and run 55min WOT then 5 min at idle, and repeat the cycle 300 times uninterrupted.
Very very few engines attempt let alone pass said test.
Downsides? Not much of an aftermarket since they're not a light duty truck application. As time went by its dried up further. Raylar engineering is probably best source these days. Worst job on an 8100 is the crank position sensor. Not an easy swap in truck, and, need to be careful about removal because the heat and O rings on early years combined to sorta make them stick in place and want to break during in truck removal. Not so bad a job to do before a swap in.
For an 8.1 you could part out an old suburban yukon, avalanche, uhaul, or rv built on the workhorse platform. Sure silverado/Sierra got em, but, pickup prices are crazy these days for trucks capable of actual work, not these piddling v6 turbo jobs woth 4.5 ft beds they build nowadays. Longevity goes well past the 300k mark if you look at trucks for sale, but, again, consider workload and maintenance.
For a 454 I'd probably look for a rusty gmt400 suburban. Don't have the utility of a pickup, so, prices aren't as up there again.
Junkyard or pull a part? Eh... I probably wouldn't want a 454 from there except as a core you're going to tear apart and rebuild completely. Just feels like problems, considering that whole risk/reward calculation.
Nothing beats being able to drive a truck and see everything function before harvesting the drivetrain and scrapping converters/carcass for parts to offset the cost.