Need to make my standoffs. Should I use 10-32 blind nuts on backside of firewall with through holes or tap standoffs and use bolts on both sides of standoffs?

I ussed 1/2-20 bolts the standoffs i have came threaded

I'm sure it depends on what type of engine and how long the standoffs are and what they are made of. In general I prefer to see them drilled through and then bolts through into blind nuts, but that's just me.

TF

Not knowing what you are making stand offs out of, or what type of engine, I still agree with Tom on drilling through and using 10-32 blind nuts on the back of the mounting box.

If you can't find 10-32 bolts the length you need, 10-32 "All Thread" works great double nutted on both ends with a little Locktite.