I've been rather disappointed with the lack of response to this posting! Nonetheless, I am proceeding and will offer the following update for those faced with a similar project problem.

I've walked my local (SoCal) Pick&Pull junkyard searching for larger snorkel/heat riser valve assemblies. The results have been rather underwhelming! There are relatively few carbed systems still present in the self service yards in Southern California! The largest Ford Snorkel inlet system I could find was on a carburated LTD. The inlet cross-section of the cold air collector is 2.2 inches tall by 3.5 inches wide for an inlet area of 7.78 sq. in. The diameter of the hot air inlet was 2.0 inches for an area of 3.14 sq. in, or less than half the cold air cross-section. I found a stripped '68 Lincoln Mark ?? that no longer sported an air cleaner, but had a heat riser pipe that was 2.65 inches ID suggesting a hot air inlet of around 5.56 sq. in. No way to verify the cold air inlet area.

The largest GM inlet snorkel I could find was on a carbed Cadillac engine and the cold air inlet measures 1.56 inches high by 5.13 inches wide for a cold air inlet of 8.05 sq. in. The hot air inlet pipe was 1.5 inches in diameter for an inlet area of 1.77 sq. in. Surprisingly, the cold air inlet area is nearly the same as the large Ford, but the hot air inlet is just about half . I found many other GM air cleaner/snorkel configurations, but all the cold air inlet cross-sections were much smaller that this unit and the hot air inlets were all the smaller 1.5 inch diameter.

I did find one interesting anomaly however. This was a lower airbox section with a cold air inlet molded into the plastic structure. The only identification decal suggested that the unit was of AC/Delco manufacture. The flapper valve was metal and was vacuum actuated. The hot air inlet was a metal tube and flange sealed and mounted to the airbox. Here the cold air inlet measured 2.0 inches high by 5.79 inches wide for a cross-sectional area of 11.58 sq. in. and the hot air inlet was 2.0 inches in diameter. Upon close inspection, it appears that this flapper valve can be ported to the Ford E350 EFi air duct I described in the original post. With some minor modification of flapper valve design it can be made to seal the hot air port that can be adapted to the Ford duct. The hot air inlet would be quite close to the original hot air inlet used on the '82 snorkel system. With a metal extension of the flapper valve surface, the effective cold air inlet area is nearly twice the original '82 inlet area. I will now proceed to build a prototype of this ported GM to Ford E350 EFi duct with integrated hot air riser or pre-heater system.

I'm still confused as to why the factory hot air inlet systems have such smaller areas than the cold air inlets. BTW, all the Mopar systems I found were smaller than the Ford and GM units described here. Other comments still solicited!