Replacing engine in 03 sierra 4.3 v6

Got my 03 a couple years ago. I been changing the
oil (full synthetic pennzoil) every 3000 miles even
though my mechanic tells me i could do 5000
miles. I figured why break a habbit. Recently we
discovered something when my low oil pressure
indicator came on that there is hardened sludge
thats starting to break up and cause problems. My
mechanic indicated this could have been there
before i bought the vehicle and ai asked him why
its why is this just now coming up after nearly 3
years he said the synthetic oil may have played a
role in dissolving and breaking apart the sludge.
Whether thats true or not i dont know but he
recommended getting a completely new engine or
getting a new vehicle all together as flushing the
vehicle will only make things worse and will cause
a hard knock.

I plan on replacing everything and getting the long
box and not the short for the purpose of getting
everything new. I'm going to get a full diagnostic
done then proceed to getting it all done. I don't
want to run into other problem with parts that will
require them to take everything apart again. I just
bought my with a 2017 chevy equinox and I'm
definitely not about to get another payment. My
only option is to get a new engine. I found one
from a company called powertrain for $1800
though i forgot to ask if that's the long box or short
box. I'd like to k get an idea of how much the cost
of labor to get this done plus anything that should
be done while they already have the engine out that
i should also look into.

Theres a chamce that the diagnostic will point to
something simpler however i want to expect
essentially the worst.

Thanks in advance.