Why did they put that effort into a small port and never grow up to the large port?
I would surmise that most of the racers in the classes that would have been blown were probably using Chrysler engines, making the market for a 348/409 blower intake pretty small. Given how few exist today of any brand, there probably weren't many sold originally and the aftermarket manufacturers probably didn't deem the expense of tooling large port versions to be worthwhile.
There was likely a similar situation on the fuel injection side. Algon catered to niche engines moreso than Hilborn, which is probably why they were the only one to do a 348/409 FI intake - and later accomodate it to fit the large port heads.
That can't be true because that aftermarket Weiand intake they milled for the base didn't come out until the mid to late 60's if I remember right. Someone should be able to confirm that.
This wasn't milled from a 2x4 intake, though. It's a dedicated casting.