How hard is it (how much work is involved) with converting old barn board into flooring?

I have done this, although I used the boards to create wainscoting on a wall. Similar process, I would think...

You can have different widths of boards, but not different thicknesses. Use all the same width board for each row of flooring.

Use a metal detector to verify no nails are present before you clean them up.

You'd have to run them all through a thickness planer, which would resurface them and make them all the same thickness.

You'd also want to run them through a jointer, which would straighten the edges and make them fit better together (eliminate gaps) on the floor.

Best to also run them through a table saw and/or router with proper bits to create the tongue and groove, as mentioned above.

Use the proper underlayment to eliminate moisture and start with a clean, level surface.

Turning an old barn into a house? GREAT IDEA. There's no need to dismantle,you can just get a building mover come over and lift the barn 6-8 feet and put it on blocks,then pour the slab. Call them up when your done and they'll lower it. Some moving companies will also do foundation work. But, before you start anything, get a structural engineer to look your barn to see if it is structurally sound. Best of luck.

no small task....

1 remove ALL nails

2 find a woodworking shop willing [many will not as nails can ruin a plane in a second.]..so look at sears,Lowe's or other place and buy a planer.. [and table,router] plane and edge the boards to your satisfaction...tongue and grove makes a better.quieter smoother floor...

You need to know what kind of wood you are using. Some can be so soft that they don't make good flooring. If that is the case then maybe you should consider using the wood for some other purpose.