Nine speed Sti conversion:
The rear mech will do 9spd, the front will need replacing if it doesnt index well with with STi (experimenting is required).
The rear stays will need widening by a couple of mm each side to accomadate the cheap 9spd wheels you'll buy (instead of faffing with old cassettes/ hubs) or the rebuilt 9spd rear wheel.
You'll need the frame stops in place of the existing downtube shifters.
You'll need a nine speed chain, cassette and sti.
Eight speed STi conversion
By going 8 spd you can still reuse the original chain/ rear mech. You will still need the rear hub replacing to fit the 8spd cassette. The rear stays will still need widening for the slightly wider rear hub.
regarding frame widening - your frame is steel, it wont even notice.
I offer this advice from personal experience of running 1988 Deore mechs on 2006 Tiagra 9 spd STi for one of the freaky bikes i make.
I've just done 8spd RSX STi with exage motion road mechs.
The anti faffing Cheese hase spoken