Swapping ratios in an overdrive A-833?

My '70 Duster is currently equipped with a 904 and cheap 2600-stall converter behind a built 360 and while it launches the car really hard I would prefer a 4-speed manual overall. I know auto-to-manual conversions on these cars can get pricey, so I was looking into possibly swapping in an overdrive A-833 since they can be found for cheap and having overdrive would be a Godsend for me. However the gear spread on these sucks, mainly the 3.09:1 first gear is way lower than I need esp. if I am going to be running 3.55+ gears in the back. Is there a way to swap a 2.66:1 first gear set from a regular A-833 into an overdrive? I don't have experience with these at all, just looking for a cheap way to be able to slam gears in my car but not give up any performance or 'feel'... I know Passon has an overdrive gear set but I'm not ready to shell out $1500 when I could save that money to put towards his new 5-speed.