Pretty sure its 18", but I know the 17" will fit no problem. I've got a 16" e-fan in the stock shroud with lots of room all the way around it.
And yes the stock fan compared to the e-fan does cost a difference of 5-10hp. Going from the stock fan to an e-fan changed my 1/4 times by 2 tenths.
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1989 Mustang LX Notch
5.0L, T-5, 1969 351W heads, cut & welded stock plenum, F-303 cam, Mac 1 5/8 full length headers, Cat'd H-pipe, 3.73 gears, Underdrives, SSM lift bars, E-Fan, 3G Alt, Custom Chip, Autometer gauges, Weld Draglite wheels , M/T ET Street Radials
Best 1/4: 13.33@101.53MPH
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