...Tom, checking the float bowl that way is brilliant...
Like so many things, this is an ancient method. It has to go back over 100 years; there is very little that is new under the sun
SU carburetors were invented 115 years ago and were known as Constant Depression carburetors. CDs are also known as Constant Vacuum (depression being another word for vacuum) and Constant Velocity. CV is the vernacular currently most used and is what the Keihin CV carbs are called.
A very long time ago, before Al Gore had invented the internet and global warming (the better to give us a means to argue and something to argue about), even before we walked on the moon, I was introduced to SU carburetors. MGAs, Spridgets, and Healey 100/4s are neat cars, but dang if they aren't hard to tune when the carbs are worn out. I soon called them Constantly Depressing carburetors based on trying to get them cars to run while I was in High School. They usually leaked both fuel and air and, later in life, I spent hours on a small lathe making bushings and glands for butterfly shafts and hours on a Bridgeport line reaming those bushings so that they at least didn't leak air. The result:
Real old-timers told stories of attaching a bit of rubber fuel line to the SU bowls and slowly lowering the hose until they found the fluid level in the float bowl. Thank goodness I have always lived in the era of clear hoses and some old-timer figured out you could use it that way.
So I can't take credit.
The overflow pipe should not be hard to do. These carbs used to have them and it is all but there. The boss that it was mounted in is still there and the nipple that the gas is to run out of is still there and still has a hole in it. It seems that all that is required is to drill a hole of the same size as the ID of the overflow tube (probably 9/32") through to the factory drain hole, then counterbore it 1/8" and install an overflow tube, cementing it in place. JB weld should do.
I think I will have time tomorrow.