Need to know a couple things for a complete answer, but that much oil (as you describe) would not come from the gearbox. So... as many do, the leak is almost certainly from the back of the engine.
Did you install an oil pump cover? If so, did you seal it well on both sides?
Did you inspect the back plate for cracks? It's not uncommon for there to be a hairline crack between the oil pump cover opening and the crankshaft. The will allow the oil pump cover to leak.
Do you have a PCV installed?
Did you have the block line bored or honed? The rear main has what's called a scroll seal. If the main cap and half moon piece had any wear, the air gap will be out of tolerance and not function as designed. If you removed the upper half (half moon piece held with 3 bolts to the block), this has a specific procedure for reassembly. If you just bolted it back in place, you didn't assemble it correctly.
Do you see any oil dribbling down the back plate between the oil pan and back plate?
Any of the above can be causes for the leak, but typically, it's the scroll seal. If all other measures prove fruitless, I make a seal kit for the rear main that works.
This link will give you some insight into the working of the scroll seal:
https://gerardsgarage.com/Garage/Te...rdsgarage.com/Garage/Tech/SealKit/SealKit.htm