This is a little bit difficult to explain but if you're worried about your existing spi halyard being a bit short with a 1:2 tack line then one of the easiest ways to maximise the length is NOT to tie the tack line off at the end of the pole.
I use back to back pulleys, one end is attached to the spi foot/tack and the other in the case of the blades 2 up Stealth's at the connection between the compression strut and pole. This has the added benefit of shorten both the tack line and spi halyard. The distance between the front beam and roughly mid spi pole is ample for the single line back to back pulley system to work.