How long is the piece of string?

I see that you must have a Multimeter but time to invest in a visit to the
auto sparky I think.

Hmm does it have a good earth from engine to Chassis? Start/crank okay?

  Your alternater is NOT charging. Your voltage reading of 11.7 volts
indicates your battery is discharging. A fully charged battery will read
around 12.6 volts.

I have a new

  You need a wiring diagram for your year truck and a multimeter. Check all
the charging system wires for continuity and proper voltage.

First of all, 11.71 volts is not charging at all. A charged 12 vdc
battery should be over 12.6 vdc without the alternator on. with the
engine running the voltage should read anywhere from 13 to 14.5 vdc,
depending on the state of the battery. I would put the battery on a good
charger and get it fully charged. I.E., 13.2 vdc or so fresh off of the
charger. Then see if the alternator is working or not.

I had an '87 that did not charge right when I bought it. We found out
later that there was two different alternators for that year and the
previous owner had put in the wrong one. Both had internal regulators.

The only thing I can think of is Grounding.

You need to make sure you have ground straps from the battery to the engine
and the body. The alternator is mounted to the engine, so it should ground
to the block, but the block has to be grounded to the battery. It is
possible tyo have too many ground points, but it is far easier to have too
few. Lack of proper grounding creates what is effectively an open loop that
is very unstable. Obvioulsy, the engine will ev entually be grounded through
the various casings and other components, but this ground is very poor even
on a good day. What you really need is a heavy guage wire from the battery
to the engine, and another from the engine to the body. Some will say that a
ground from both the engine and body to the frame is also a good strategy
because there are body mounted components that are physically grounded to
the frame.

Since you just replaced the motor, I have to wonder if the ground straps
were attached. If you have to attach ground straps, I suggest stranded
10-guage wire (as opposed to solid wire). You can go as heavy as you want,
and I recall the battery cables are something like 4-guage -- where lower
guage numbers equate to heavier weight wire. Heavier is better, and good
connections are a must. Clean metal is far better than painted surfaces.

One other thing I can think of is a defective alternator. You can remove
this component easily enough, and they will test it at the auto parts store
for free.

I had the same problem with my '85 truck.
Turns out I got a bad rebuilt alternator.
The second one worked fine.
Take the "new" alternator back to where you got it and have them test it.