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My friend and I are currently converting a 3B to veggie and I've been thinking alot about the valve configuration.
First, the pump will push much more fuel then the engine needs even under wide open throttle, in fact, via the engine manual the feed pump should be providing .9 L/min. This is compensated for via the return/leakage pipe on the injectors which goes back to the pump. The key for fast purge times is to eliminate this return flow via a 3-port valve at the rubber portion of the return pipe, off the no. 4 injector. Also, having two 3-ports allows back-flushing of the veggie line with diesel so that when you shut down no portion of the line not exposed to coolant heat is filled with SVO.
I think that we are going to splice in the supply 3-port just before the feed pump and allow the stock filter to be exposed to both diesel and veg but have second dedicated, heated filter on the veg supply side as well (this filter is pre-valve). I would NOT bypass the stock filter after the feed pump even with a second filter pre-pump, Toyota put it there for a reason which is most likely that the feed pump itself will create metal shavings and junk that isn't friendly to IP (even inline ones). The rule of thumb is nothing but fuel line should be between the filter and the IP.
With the config. I've outlined I think that you'll find your purge times to be much faster. I have about a 40 second time on my 2LT-II with pretty much the same setup. On shut down just hit supply to diesel and leave return on veg to back flush to the filter and eliminate any oil in the lines. I've think that with 2 3-port switching over is nicer as well as the fuel is slowly mixed when you first hit the switch as returned diesel mixes with SVO. Given that it sounds like you'll be doing the same drive repeatedly, you should be able to get a feel for when you are up to temp and may be able to "cheat" by anticipating when you'll be on pure SVO making your switchover times that much shorter.
Hope this helps,
Later...
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