I've owned a couple of Corvairs, which have log intake manifolds that are integral with the aluminum heads. What Corvair people have been doing since the Sixties is cutting off the top half of the log manifold, welding a top plate to accept multiple Weber carburetors. Doing so to an Alfa L-Jet would be similar.
Assuming you live at sea level (easier carb tuning), have or are going to install performance cams and pistons, it seems like you could convert an L-Jet to dual Holley 2 barrel carburetors with some relatively minor fabrication.
-Using the original L-Jetronic plenum, I would simply cut horizontally in half..... or pretty much in half.
-Next install tubing to extend the cast intake runners to just above cut line.
-Cut aluminum plate to cover plenum, with holes for extended runner tubes. Weld cover to original plenum.
-Fabricate dual Holley carb mounts, orienting linkages towards center of car.
-Fabricate mount to support stock L-Jetronic throttle linkage.
-Fabricate linkage to connect L-Jet linkage to Holley linkage.
-I don't know if the carbs will stick above the hood line, but if so, a tear drop blister similar to a 250 Ferrari Testarossa might look good!
SUPERCARS.NET - Image Gallery for 1958 Ferrari 250 Testa Rossa
Holley makes two 2-barrel carburetors; a 350 CFM and a 500 CFM, both for under $300 each. A whole lot less expensive than Webers.
Conversely, it seems like a cheaper way to go from SPICA to carbs using the L-Jet plenum, rather than Webers.