a couple of things ...
if all you have are 12v fans you could probably run unregulated ok. (i haven't tried it either, just going by what mini-box says) ... the fan will just run a little fast and may burn up quicker
I don' tknow the min/max voltages for the 5v & 3.3v line on the pw-70. If they are regulators, it would probably be somewhere around 6.5v and 5v respectively. But I don't know what method they use on the PW supplies to step the voltage down. Haven't looked into it really.
Putting the tank between the PW-70 and the mobo won't do any good. When the car cranks the PC will still reboot if the PW-70 gives out. Your PC wil still get 12v from the tank but the 5v and 3.3v lines will die. So your HD and Processor will go with it.
And when the car's running you'll still get 14v at the mobo from the alternator.
putting the Tank in front of the power supply ensures the PS will always get at least 12v during crank, allowing it to properly supply the mobo.
you could maybe place the tank in between the ITPS and the PW-70 .... but then if you use the PC when the car is off (only getting 12v from the batter) you'll get about 11.5v on the other side of the ITPS and your tank will have to make up for the difference. depending on the Ah rating of your SLA you might be ok for a while ... but then it has to recharge more.