need a quick answer here guys, how long will a 454 and 350 last in terms of mileage. potentially can get for cheap but have around 100k on them.

depend on how you treat them or the other person treated them, as with any motor.

100,000 is barely broken in. (If the oil is changed often enough, etc etc)

I disagree.
~100,000 miles on a 70's motor is quite a bit.

On a modern motor--no issue...but in the 70's the machining and fitting of internals was not as precise as todays standards. Tolerances were looser simply because they couldn't accurately make them any better. Materials used were also not of the same quality we would find today.

With 100,000 miles on an "average" 350 or 454, bone-stock, never rebuilt, from the 70's will have some blowby, uneven cylinder pressures, and will leak/use some oil....almost without a doubt.

Not to mention you're talking about engines that have survived over 30 years of use, abuse, sitting, etc. You can't say how long they've sat or how well they've been maintained.

Want to know for sure?
Do a leak-down test on the motors in question, that'll give you a good idea on their wear.
Pull the intake and/or drop the pan...look for sludge, metal debris, or even pop some caps and inspect bearings.

In the mean time, look at them as "cores" for a future build.

The thing is , how believable is 100,000 miles on a 76 motor?

X2, considering that the speedometers only went up 5 digits on that year range of trucks.

Could be 100,000, could be 300,000.

I got over 250,000 on my 454, and its still running.

What I'd ask is why it really matters... They run? Can you see them run? If so, make sure she don't smoke and the oil ain't full of metal. Beyond that, fuck it, rebuild it when the time comes. It won't be overly expensive.

take the big block. run it as long as you can, and then rebuild.

Drive it til she blows, rebuild, then repeat. :flipoff2: