I recently made a Single F Alto Horn out of an F. E. Olds Bb Horn and a Conn Mellophone (first picture).  I used the valves from the Bb Horn and the bell from the Mellophone since it was a slightly more compact wrap, yet still had a throat that was comparable to a Horn.  Aside from the slide crooks and a few of the braces, I made everything else.  In the second and third pictures, you can see the finished product.

As you can see, I changed around the arrangement of the valve slides a bit to make the wrap a little easier to deal with.  The leadpipe enters the valve section in what would normally be one of the first valve slide ports and the first branch leaves the valve section through what would normally be a third valve slide port and the slides for said valves go out the sides of the valve section.  The advantage of doing it this way is that the tubing is easier to bend and therefore allows more length to be dedicated to the tapered parts.  If the leadpipe and first branch entered and exited in the normal way the amount of taper would have been much shorter and therefore faster, which changes the playing qualities.  The leadpipe would have had to end earlier at a cylindrical offset tube to bring the tubing around to the front of the Horn and the tuning slide would have been shorter.  The first branch would also have been shorter. 

I actually maximized the amount of taper in the leadpipe by turning down the leadpipe tubing at the end and boring out a piece of slide tubing so it would slip over the leadpipe.  The pipe is therefore actually tapered inside the slide tubing and ended up with 15 inches of taper with two inches of pull on the tuning slide.  By contrast, most High F Horns only have 8 to 10 inches of taper for the leadpipe and usually have a tuning bit at the beginning of the pipe rather than a real tuning slide.  On top of that, the leadpipe on this Horn doesn't interfere with stringing the valves and makes it quite comfortable to hold the instrument.