I am forging 3 mantel brackets out of mild steel for a 99"x 9" x 4 1/2" old barn timber that weighs 90 lbs. What type of hardware would you recommend to secure it to the studs? Each bracket will have 2 holes on the vertical run to attach to the wall/stud and will be 1/4" thick. I can get #20 (5/16 shank) pyramid head wood screws that are 2 1/2" long. Or, should I go with a 3/8" to 1/2" lag bolt? What would be an ideal length for these?
I think your choice of 5/16" size lag screws, 2-1/2" long, is fine. I would say 3/8" is overkill but if you just like the look of the bigger fastener, it would be fine.
I'd actually use something a little smaller, but longer - 1/4" size, and 3" long. (And washers, whatever size you use, I'd use washers.) One of those is adequate to support 90#. I'd also drill a pilot hole, 3/16" bit works well for 1/4" fasteners in most wood.