Going from east to west is not as easy as going in the other direction.
The east west direction is against the current
(above the seaway locks), against the prevailing wind
and up locking in a seaway lock can be an awful
experience. The locks are filled at such a tremendous rate that boats get smacked against the walls of the lock. Have lots of good big fenders and maybe some spares along too. We popped 6 fenders in one whack. I suggest you ask for a slow fill.
However, many yachts do it and if played correctly its a fine trip.
The key is to pay close attention to the tidal currents and use them to your advantage. To pass through the Richelieu Rapids can only be made with the in bound tide when going up river.
The best book around with lots of information for pleasure craft traveling the St. Lawrence River is called Cruising Guide to the Down East Circle Route
by yacht pilot publishing. The circle route takes a clockwise direction but the book gives information for river currents for up bound yachts too.
There is a good FAQ page on their website Welcome to Yacht Pilot Cruising Guides
We've cruised the St. Lawrence River in both directions many times and really enjoyed it.