To endorse "Goldenrust" comments. If you are going to take the engine/gearbox out, do them seperately. I am closer to 77 than I am to 76, and I can heave an engine out from starting to put it on stands 'till it is on the floor in 1-1/2 hours, but I must admit, it takes a wee bit longer to put it back! Make sure that there is a square of ply (or similar) between the jack head and the sump. Don't forget, if you are going to heave the engine out---disconnect the battery first.
To, again, endorse "Goldenrust", there are only 4 places that the gearbox can seriously leak oil; the 2 drive-shaft boots which CAN be done with the engine and gearbox 'in-situ', the gear-change rod seal at the front of the gearbox which sometimes can be changed 'in-situ'. I say "sometimes" because some gearboxes have a seal that can be removed externally, other 'boxes rely on 'O' rings inside the rear gearbox housing to seal the shaft. Lastly, there is a seal/bush unit INSIDE the bell housing through which the spigot-shaft passes through. If this is the cause of your leak, then the engine will have to come out. The little housing CAN be removed with the gearbox still in the car----but renew the housing COMPLETE (Mr Fiat, in Atlanta, part number---TM-500-047)---removing the seal from the housing can be a "reet booger" of a job! If you are not suffering clutch-slip,I would have thought that the chances of the 'spigot-shaft seal' being the cause of your leak is slim. When you last filled the gear-box, how much oil did you put in the box? The "fill" plug is also the "level" plug---you need to have the car as level as possible when topping-upthe oiland let it drain until it stops coming out---and make sure that the fill/level plug is tight when put back in (you wouldn'tbe the first person to have onlyput the plug back "just over finger-tight"!
To go back to your carb problem---you say that it is so rich that the car smells of petrol----what sort of fuel-pump have you got fitted? Is it the standard pump on the side of the crankcase, or has the previous owner fitted an electric pump? I ask this because the fuel-pump pressure required for the 500/126 engines is only about 2-1/2 psi. The float level should be 7mm; measured by holding the top of the carb vertical and measuring between the gasket and the float. The easiest way to measure the float height is to use a 7mm drill-bit (or the imperial equivalent) between the gasket and the float.
Lastly, could the pump be OVER delivering (if it is the standard mechanical pump). The pumpis actuated by a rod internally coming across the crankcase from the camshaft (small lobe on camshaft) to the pump. This rod should only stick out past ALLthe gaskets and the spacer 1.0mm to 1.5mm at THE START of its movement. The easiest way to measure this is to build up your feeler gauge to 1.5mm (60thou) and then put this against the pump's operating rod---the end of the rod and the feeler-gauge should be level.
