Vince,
Blue Thunder manufactures a 2 plane manifold which is available with a "flat mount" carb mounting pad. It is the only manifold I am going to mention that has a flat mount carb mounting pad. It is a very good replacement for the stock manifold as it opens up the ports which are pinched down in the oem piece. It is aluminum, designed only for the large port heads, designed only for Holley bolt pattern carbs (no spread bores). It raises the carb about 1 inch vertically, so clearance beneath the engine screen becomes tighter. Air filter bases designed for the late 60s big block Corvettes allows fittment of open element air filters with this manifold, the air filter bases are available from Corvette restoration parts suppliers, such as corvetteamerica.com. This manifold has a very good reputation as far as performance is concerned, rivalled or bested only by the Holley Strip Dominator and the high rise single plane intakes such as the Parker FunnelWeb. It is available from some of the Pantera vendors, such as Pantera East.
The Weiand Xcelerator for large port heads (part number 7517) & the Edelbrock Torker both have poor low rpm response and are not recommended. For a large port single plane manifold, that keeps the carb mounting pad about the same height as the Blue Thunder your only choice is the Holley Strip Dominator, which is no longer manufactured but can be found used. This manifold gives up some low end torque compared to the Blue Thunder, but provides additional horsepower at upper rpm.
For small port heads, the Weiand Xcelerator, part number 7516, is the best choice, outisde of the high rise single plane manifolds. It also raises the carb vertically about as much as the Blue Thunder. It is a single plane manifold, but delivers smooth operation at lower rpms. The carb mounting pad is NOT machined flat like the Blue Thunder. Edelbrock shall be introducing an RPM Air Gap manifold for the small port heads next year which I am sure will be very competitive with the Weiand Xcelerator. No info avaiable yet on that piece, the pictures I've seen make it look as though it raises the carb even higher than the Weiand, but looks can be deceiving.
I have avoided delving into the high rise single plane manifolds, such as the Parker FunnelWeb, because you mentioned keeping everything below the engine screen. I am not supportive of any Offenhauser manifold, or of the Edelbrock Performer manifolds, hence my silence regarding them.
George
