Dave, I'd stick away from that particular carb spacer. I'd think you're making too drastic of a change from the carb to the 4 hole. Also You may have issues with your vacuum routes on the carb.
I'm curious to see how the 390 works out. I've been considering them for mild builds, but I always head to the 600 cfm range.
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93 Thunderbird 5.0 HO
89 Firebird L03
95 F150 XL
Get ready for the 351 boss build this summer, 4v closed chamber heads, 2.25/1.75 valves, .750 lift, 12.5:1 piistons, wieand tunnel ram, nitrous, I think its gonna roar!
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