Sandblasting (garnet) under the floor is fine, but take care never to use garnet on  surface top panels as the process can stretch and distort panels. Often there are grade choices of sandblasting, plastic bead blasting is also popular. I think the soda blasting is the new big thing.
Chemically removing or mechanically removing the underbody treatment is also another way to go.
Originally a soft sound deadening rubber based paint was used in selected areas and then painted over with the top coat.
If you happen to be at Winton this week-end feel free to make yourself known and have a look at my car, or alternatively at other club days. I hope you might consider coming on my EMR on Sunday 31st in the Dandenongs.

Clive

Just in relation to your other queries re the resto of your GTV.  If you were going to subject the underneath of our car to any of the above mentioned treatments, it might well be preferable to remove your suspension given that there are a range of rubber bushes which could potentially be affected.  On the otherhand, if were possible to sufficiently mask off and cover them up that probably would be ok. I would check with the specialists who do this work to seek their advice. 

In terms of repainting the underneath after treating it. From what I have read elsewhere, including the Alfabb site, they all seem to recomend resealing the bare metal with a primer surfacer, then a stone chip paint and over that paint the body colour.  105 Alfa's like many cars of this era were painted the same colour as your body shell, underneath.  How far you wish to go, depends perhaps on the standard you wish to achieve.

Re rust proofing, I have seen various treatments come and go over the years.  Some recommend POR15 but while effective in killing off rust it is difficult to paint over, probably the best bet is to either sandblast, beadblast, wire wheel, acid dip (depends on what areas you are treating, then etch prime and asap use a primer surfacer and then a topcoat.  I have previously used rustkill on interior surface areas in other cars I have restored but this was in my earlier days.  These days I would spray and internally thereafter re-apply with tarpanels on the floor areas for instance, but not if I were building up a race car (lightness being paramount).  The other thing thing to use for cavaties (ie. cills etc) is some form of other treatment such as fish oil (though you will need to re do this treatment everynow and again), or a wax treatment such as Waxoyl.  The Poms love this stuff and swear by it in rust proofing their cars.  I am sure there are more options out there which perhaps others might be able to help with.