Soffit construction questions

So i've got 1 wall finally framed, and on to the next one.

The problem is that i need a soffit to surround the main drain pipes.

A contractor suggested i could build a 2x4 ladder frame attached to the joists.
and then take 2x4 from that ladder to the wall, and attach the top plate there.
And do the usual attaching studs to that top plate and a glued/screwed bottom plate.

SO the top plate is only held into place by the L shaped soffit.

Would that be sufficient?

Sorry can get pics later, but is it ok to penetrate the foam insulation, that i spent so much time making air tight?

Or just seal around the holes made?

And sound proofing isnt an issue, what should i use to attach top plate to concrete foundation wall?

Ok so i got some pics... this is the wall in question, its 16' long and 8' 2" tall



Here is how I wanna attach the top plate... picture that horizontal 2x4 being the plate, and yes it will be level



Here's a side view



What do you guys think, will that be sturdy enough?

THen i'll put a ladder frame outside of pipe, and ledger on wall and sheet rock across..

Its hard to get a good solid screw in that angle, i'm hoping one every 3' would sure it up.

And that foundation wall is plumb as can be. phewww...

BUMP, any opinions???

Yeah, thats one of 1000 shortcuts KBHomes took on my house....