I'm a big fan of slants with three-on-the-tree. You are correct, the A903 top loader trans sucks the big donkey dick, and completely due to the non-synchro first gear. I have one in my 66 Valiant and I hate it. And this is coming form a guy who's spent the last 50 years rowing gears on both a personal and professional basis. I can shift with the best of them, but the A903 first gear is complete crap.
The easy way around this is to get an A230 3-speed and swap it out. It is MUCH stronger, and has all synchro gears.
If we were a year or so down the road, I'd share how I'm going to turn my three-on-the-tree into a four-on-the-tree. It will be theft-proof from any millenial on the planet. :p