Way to heavy a spring on the rear!!!! Must be a real back pounder on the road!!!! I'd drop down to at least 250, maybe 200. Last coupe I built (it was a featherweight) I ende up with 180# springs on the rear.. But, that was with an aluminum SBF with aluminum heads, and ultra light everything. Even the 4 bars and front A-Arms were moly... Yours should do good in the 200-250 range.