Got the New Tank...working on plumbing

Allright, As I stated in another thread we have made the move to a 75
gallon tank. We have the tank, stand, and all the accessories. Now
comes the joy of putting it all together. In the final analysis the
sale wasn't as good as we had been told, some Jr. fish sales person had
said that everything was 50% off...but it turned out that fish were 50%
off. As a result, we decided on a single Fluval 404 rather than two
304s. We lose some redundancy, but saved some money.

What I'm planning to do is connect the output of the 404 to an under
gravel filter creating a RUGF. The UGF I has three 15"x12" plates that
can be configured as 45x12 or 36x15. The tank is 48"x 18. I'm
planning to punch out the flow-through panels on the sides of the
filter plates to create one large filter plate (45"x12). I've slid
that toward the front of the tank, to provide an area at the back of
the tank where there will be no UGF. Our thought is to heavilly plant
that area, and possibly mix in some finer substrate at the rear of the
tank to better accomodate the plants. A plastic divider will be used
to provide somewhat of a barrier to prevent the finer substrate and
some of the roots from interacting with the UGF. I'm also hoping that
by punching out the flow through panels, the output of the 404 will be
somewhat evenly distributed beneath the UGF plates.

On to the plumbing... The input for the UGF is a 3/4" hard plastic
tube. I'm planning to use a 3/4" PVC slip/slip elbow to run a 3/4"
pipe to the rear of the tank where another elbow will connect to either
a horizontal PVC riser or some vinyl tubing (either 5/8" or 3/4".) I
haven't tried any of this yet, but I think my 5/8" vinyl tubing ought
to slip over the end of a threaded 3/4" PVC adapter if the tubing is
heated. I plan to top those off with a hose clamp, at least when
making those connections outside the tank. Alternatively I've seen
reference here to folks joining 3/4" vinyl tube with 5/8" using a
combination of heating/cooling and PVC cement. As the filter itself
uses 5/8" connections, this transition is somewhat important.

I figure that this is a pretty common issue, so someone can hopefully
tell me if any of the above proposals will, literally, "hold water."

Thanks in Advance