Crumb, if that rattling all the way through your video is the fuel pump, it sounds like it isn't getting a good enough head of fuel to me. I had a small facet pump on a 1200 wagon a long time ago (1994 or so) that did this, it wouldn't rev as well as it should have, (A14 with 70 degree cam & twin springs & a 32/36 DGV weber) that wouldn't break into the 16's at Eastern Creek street drags, it'd run out of revs at about 6000 rpm. Was doing 17.0's all night.
Re-read the instructions for the fuel pump the next day & found the fuel pump should be mounted at a 45 degree angle. After remounting it the motor went straight round to 7000 rpm, like it should have. Never ran that car again but it was appreciably quicker to me. This might be the way to break into the 13's. If you have the facet pump like they use in the series 2 B/Bird, it has to be verticle, not horizontal, with the fuel outlet above the inlet.
There's always the posibility that Im wrong, but all my mates Hillclimb VW's used the small square facet pump & they all had to be mounted at a 45 degree angle with the inlet lower than the outlet. This was enough to set class records in the under 2 litre 2E/3J club car class in various cars, & win 2 Australian titles in that class.
We always measured the fuel flow & it had to be 2 litres or so per minute or the flow wasn't enough. We just stuck the fuel hose into a 600 ml bottle & ran the pump for 15 seconds. If it went close to filling the bottle, it was flowing fast enough.