Why is it that a Bosch O2 sensor will last 100k-200k miles, and probably 15+ years, in a new vehicle, but seems to fail fairly quickly in any aftermarket setup? And these cars are typically not being driven in snow and rain. It seems odd to me.

I think its because almost all new cars use a narrow band O2 sensor, not a wideband.  The narrow band sensors are probably more reliable.

The new LSU 4.9 sensors are much better than the old style.  Looking back at sensors going bad on me, I'd say not letting all the moisture in the exhaust cook out before powering up the sensor was likely the biggest mistake I was making. 

If the dishes and valve relief are the same, flip them upside down and check pin height.  Easy to do on a hard surface, line up pins and see if they are the same from one to the next, building a bridge

If it's a 1.76 compression height piston (for all eight) I would expect it to be 10.138 or so total stack height, which would leave you right about what you see with a 390 crank.  .035 in the hole is pretty deep, many have run there but compression will be low and it needs to be because quench will be poor. 

If it's a budget engine, which I assume with used pistons, and you want to get around, it'll do OK assuming you don't put a cam too early or small in it and watch the timing curve.  I did one like that as a temp motor and it ran very nice, not fast but a good runner.

If you wanted to get a little fancier, cut the deck to 10.150 and run a Felpro 1020 and you'd be closer to .057 quench, that's about the top of good quench, but not bad quench either.  Then cam accordingly

Well fingers crossed

Joe, I have an edelbrock performer rpm intake for it. It'll look almost the same as it did previously, but I think I'm going to leave the intake unpainted this time and now I have a Hipo air cleaner for it. I think the trick flow heads should work well! I had Jason Anderson go through them and fix them up so they don't back up anymore. Nothing wrong with the trick flow heads, just feels a little wrong to bolt someone else's parts on it when we make our own that flow better. I'm just getting over-eager to get the car on the road this spring. We're getting over the final hurdles with our heads, so there's always a chance that those are ready before I drop the engine in the car.