You don't mention how you plan to drive the 74154 4 line to 16 line decoder?
Given a choice I would drive the decoder using a 74193 4 bit up/dn counter. Then control direction on the counter using a simple SPDT switch (or electronically). I would not use relays but would likely use the 74154 to drive MOC3010 (
data sheet to be found here). Get a bunch of SCRs rated for maybe 400 V and a few amps for the MOC3010 to drive. The data sheet covers it.
<EDIT> Oh before I forget, drive the 74193 with a simple 555 oscillator for adjustable speed. </EDIT>
Ron