Evening, ready to fit my chopped original cast sump (thx Ian @ Flash Customs- great bit of welding).
What's the best,if any, sealant to ensure it doesn't leak???

Mine was steel and migged, which I water test once welded and seemed fone, but just to be sure I powdercoated it, which I think is pretty good way of ensuring a seal.
I wouldnt want to use any 'stick on' sealant in case the oil broke the adesion and ended up with a strip of sealant being sucked round the system

normal black silicone is what i use.

alot of standard ford sump are gasketed with black silicone nothing to worry about

You need a special sump sealant, halfords sell good stuff made by wurth which is not stupidly expensive.

Do not use household stuff, oil will attack it ending up with a sticky leaky mess.

Use the right stuff and not to much
I had to strip an engine after the previous builder used to much grey silicone which blocked the oil pickup enough to drop oil pressure

if the welding's as good as you say, it won't need sealing

personally i wouldn't put anything inside it, maybe a good few coats of thick strong paint on the outside, and if you see any oil drips on your garage floor, then check it out

THeres a difference between sealing a sump gasket and sealing the welded edges at the base.

IF its a gasket sealant.... loctite 5910

Sorry I was not clear, I meant gasket sealant.
Thx for replies so far.

If its a silvertop sump you only need the specific Ford gasket (don't buy cheap pattern ones, the material goes soft when oil touches it) with 4 small blobs of suitable sealant at the points wehre the front and rear covers bolt to the block, there is a small step and the sealant ensures there are no leaks.

If its a blacktop the aluminium section requires Ford gasket with the 4 blobs of sealant but for the joint where the steel section bolts to alloy section there is a correct Ford spec sealant.

Do it once and do it right, its far easier out of the car.