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but it does fit inside a stock shroud right? i measured the stock shroud to have roughly a 20" opening.

Yes. I use a stock 26" shroud with the 18" flex fan. It fits perfectly. Try removing the electric fan first before spending money. What rpm do you idle in gear at? Should be around 600-800 with a mostly stock engine, correct.

4 speed street legal. Best time 10.99 @ 124 mph on 93 octane pump gas @ 3926# total weight