That's what Frankenbikes are all about, using what you have and I like this one. As for the levers, have you tried reversing them so that the cable's point towards the rear of the bike? That's what I usually do when mounting MTB levers onto bullhorns because it shortens the cable length and cleans up the cable routing. Reversed levers may take some getting used to but they feel pretty natural after a short period of time.

**UPDATE**

Wait a sec... I just realized that those levers look a lot like in-line levers. If so you should be able to leave them mounted the way you have them but feed the cable in the opposite direction.