If you're still using the conventional straight axle and leaf springs, the cowling crossmember is definitely still needed. It braces the frame where the rear spring hangers are attached. If you've butchered up the frame with an IFS of some sort, the cowl crossmember probably isn't all that important- - - -the geometry and stability of the original frame has been permanently compromised anyway.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!