Here is a pic of the sump we use, we've ran the tank as low as 1/8th full on the gauge with no problems. We generally keep the tank 1/2 full for consistancy and the extra wieght at the rear but the safety aspect of so much fuel (10 us gall?) is always on your mind. Btw some fuel cells come with the option of a level sender which can be hooked up to a autometer gauge. 
'74 Challenger..9.46 @ 145.9 1/4, 6.001 @ 118 1/8 so far. 4023lb !!! # N/A, Marsh performance 655ci, Indy Maxx, T/R, Indy 600-13 X's, Street legal, pump gas, full interior, Cal-Tracs, mufflers, 3:73's and real 10.5 radials.
9.51 @ 142.4 1/4, 6.003 @ 114 1/8 with our old mule KB, 572-13, 580 wedge.
RHD '68 Barracuda Fastback 323ci street/strip. Best ET 13.88 @ 99.03
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