350 chevy compression/performance question

I have a 66 Impala SS ragtop with a small block chevy 350 and Turbo 350
auto tranny.
I just like to drive it around now and then and go to a show.
My motor is making some very odd noise that we now believe to be in the
bottom end.
The few who have listened to it have not had a solid guess.
The last guess was "maybe it is a wrist pin"
Sounds like the lifters do not quite oil up.
The noise is present when I let off the gas and the rpm's start to
decrease.

I have replaced the cam/lifters/valve springs/push rods and rocker arms
but the noise is still present so that is why I now believe it is
bottom end.

A local shop wants $825 to completely re-do my short block.
Due to the wifes '67 Camaro taking up our money right now, $825 is a
bit steep at this time.

A guy I work with has a 350 with approx 5,000 miles on it that he will
sell me for $400.
The issue about this motor is he stated "it just has some low
compression pistons in it so you can run any gas without ping"

So,,, if I just want a short block that is known to have no issues but
has some lower compression pistons (no idea what compression exactly)
for a decent price do I buy that or do I really need to wait until I
can specify the pistons for a higher compression?

My current pistons are just some flat tops that I have no idea what the
compression rating would be.

My cam is a new comp cams 12-242-2
int exh
adv duration 268 280
dur @ .050 224 230
valve lift .477 .480
lobe seperation 110
the heads are the old style camel hump heads that were gone thru a few
years back.
my intake is your basic Edelbrock Performer RPM

So, I will listen to anyones input on the "buy the unknown low
compression motor for $400" or "wait til you can specify your
compression"

If I do buy the $400 motor, my new cam, lifters, valve springs, intake,
heads will all go onto that bottom end.

I want to get this noise issue resolved so I can have my 40 series
flowmaster mufflers finally put on.

Thanks Much.

rick2950 at hotmail

With the gas available today I would go with the lo comp motor. KEEP the
other motor if it is the original, but it doesn't sound like it. If you
have a good machinist you could have them machine .050 off of the head to
compensate for the lost compression. You will have to custom fit the
intake manifold either by cutting the intake side of the head or the
manifold itself. Your other option, depending on condition of the rest of
the motor? remove it ,turn upside down, replace the crank & bearings, a
good quality oil pump & a new timing chain to go with the new cam