Great question, Slash. My response:
First Choice
21. Spencer Fano OT Utah
53. Emmanuel McNeil-Warren S Toledo
Backup Plan
21. Blake Miller OT Clemson
53. Keylon Rutledge OG Georgia Tech
I did not select Jacob Rodriguez, ILB because I don't think he makes it to 53. This group has two very good ILBers, Sonny Styles and Rodriguez. That's it. If Rodriguez is there at 53, definitely take him.
Fano is a good RT, very athletic. Not the physical specimen you would want this early (wingspan not great) but this draft has a ton of guys who would be good 3rd round picks in years past who have moved up to 1st round options due to the paucity of genuinely great prospects. The guys I really want the Steelers to get - Tate, Downs, Thieneman, Freeling - don't have much chance of falling to 21.
The backup plan gets a potential LT with very good physical traits and an OG who is superb. The mock simulator I use has Rutledge falling to 53 about half the time so heck yeah.