Please could someone tell what the oil capacity should be on a stock 383 ? Thanks

4 quarts plus oil filter is what my Cuda used. what vehicle and pan do you have? trucks typically have a larger sump to hold more oil. if you don't know what pan you have, just add oil till you read it on the dipstick

Just noticed you're in London. that's 3.9 liters.

filter capacity/pan type varies (maybe on the pan type) so on the first oil change after starting/shutdown/time to settle I'd slowly add however many full/partial liters to where is is right at the full mark on level ground. that'll make it easy to spot any oil burning/leaking if the level dips below the full mark and next time you'll know exactly how much it takes. I'd start with 3 liters then creep up on it