Im currently building my first fixed gear out of my old huffy strider 10 speed road bike.
My question come to finding my bottom bracket the bracket on the frame measures at 70.671 mm and is smooth with no thread as the old bb was one piece crankset with bearings around it. My question is what size bb should i go with and what spindle length as this is the only thing confusing me on this build

BBs are fairly cheap and a decent bike shop will likely have a number of them in stock anyway. Take your bike the your lbs and have a chat, then bring home the right part

This is a good test of a bike shop too. If they sneer and act like they don't care, move on. A good shop will know that although he's only selling a $30 bb for an odd ball build, you'll come back for bar tape and tubes and lube and, one day when you're feeling rich, a new bike. Build that relationship with the easy stuff - my lbs looks after me, is happy to order in odd stuff when available and gives me $10 discount on everything I buy.

You have an American sized bottom bracket, like on a BMX bike. There is an adapter so you can use Euro bottom brackets just google it you'll find one. Or you can use the original bottom bracket and crank and change out the chainring to a single ring.

You have an American bottom bracket but can also use an American to Euro BB adapter and install any modern 68mm English-threaded BB.

edit: beat to the punch!

I was sure this thread was going to be asking to list out the parts in someone's build on youtube

Your new crankset will determine the spindle length, usually. You might have to adjust for the adapter that you need to use.

All the adapter does is put Euro-sized threads into an unthreaded American BB shell. The spindle length needed for whatever crankset you use will be the same.

Even easier.

Doesn't exist without a video in teendudespeak listing all the bits

Give it a rest, we don't need another one of those.

Good luck, OP. Gracias veteran dudes.

Awwww daaaaaaaaad