DIYbanter - Oddball unisaw guts question

Recently picked up a 1951 vintage Delta Rockwell unisaw and am
going through it before putting it in service.

In my small shop I will be welding in addition to woodworking
and we all know that sawdust and sparks don't play nice together.

I want to fabricate a blade shroud for the unisaw to catch most
of the dust as close to the source as possible.

On my saw, there are two threaded holes that look like prime
candidates for mounting a shroud of this type (they move with
the blade). They are shown in the following picture--they are
center to the picture (one above the other) and the circular
area around them has been faced flat.

http://www.drizzle.com/~dantzler/dropbox/saw3.JPG

My main question is can any one tell me the thread spec for these
holes? Secondary to that, if any one has done this before (I know
folks have since I humbly admit to not coming up with the idea on
my own) would you have any suggestions on implementation?

Some of my thoughts...

I would prefer not to put any more holes in the saw cabinet.
Most likely I'll run some kind of flexible duct (maybe even shop
vac hose) from the bottom of the shroud out the cabinet and to
a good shop vac now (and maybe dust collector later). I could fab
the shroud out of mild steel, but that would make me slightly
nervous because I shudder to think what would happen if the thing
ever came loose. I may take this as a good excuse to start doing
aluminum TIG and make it out of 6061.

Thanks for helping me brainstorm.

Jeff Dantzler
Seattle, WA