I picked up used 3 piece ISMA spoiler. I have only two bolts for the center.

I was thinking SS sheet metal screws for the ends and two by the lock.

I have to fit it soon before paint.

Drill holes and SS sheet metal screws is that the way to go.

One like this

That's a good way, but look carefully at the outer mounting holes. They're threaded.

looks good, this is off topic a bit, but were those fins on the roof suppose to help with the turbulence at the rear of the car?

That is not my pic i was just showing the type of spoiler.

I was reading some were about those and yes they spoil the drag.

Those are vortex generators and are used to break up air flow (they're positioned 8-15 degrees off center and staggered). The non-laminar flow helps the rear spoiler do its job better. The VGs seem to be best suited for a high posted wing. Not period correct or legal in SCCA, if memory serves me right.

Black Gold Man here are a couple pictures of my same spoiler.

On the side pcs. there is at least one threaded stud that mounts inside on the top, this is very hard to get to and hard to get the nut on to.

I used SS metal screws as you can see on the side and the top in the middle.

I also painted the holes the SS screws screwed into and I also put a line calking all along the edges of the pieces to seal them. My spoiler has been on since 1979. Hope this helps.