Modifying A Tailcoat Pattern

The closest pattern I prefer is McCall's 8701 (out of print, but still easy to find). It looks like a lot, but it's really quite workable.
  1. Take the notches out of the collar, and (if you like) go ahead and widen them. Just draw your new shape right onto the pattern (and don't forget to include a seam allowance). Modify your lapel lining to match.
  2. Reduce the tails and curve the ends. I recommend just bag lining them individually before attaching them to the jacket.
  3. Remove the extra seam in the sleeve so that it's just one piece. You can literally overlap that pattern pieces and tape them together as one piece. You could also just use a basic sleeve from another pattern you like.
  4. Add a center back seam (don't forget to add your seam allowance).
  5. Shorten the torso a little, There are lines where to lengthen/shorten the front and back pieces.

So even with all those changes, here's why I like this pattern:
  1. The way the (black sequin) lapel opens onto the front and still has the gold sequin underneath.
  2. The way the notched collar is faked anyhow, so there's no real piece to account for, just the shape.
  3. The easiness to shorten the torso (if needed).
  4. The easiness to modify the back and tails.

Edited to add:

How to combine a two-piece sleeve pattern into one.  It's imperative that the shoulder seams match (the seam allowances will overlap) for the sleeve to still fit into the arm hole of the jacket.  Also, I recommend overlapping the wrist a bit more for a more tapered fit.  This alteration will leave you with a seam down the back of the arm.  All of your shoulder arrows will still match up on the jacket when you set your sleeve in.