A833 O/D gears in an A833 1:1 ?

For the sake of getting exactly what I want, I've decided that I'm building my own transmission.

I have a set of O/D gears, gear cluster for the countershaft and 23 spline input shaft. The O/D uses a different diameter countershaft and main shaft is also different.

I know that the gear sets can be changed between O/D and 1:1, with main shafts, does this also mean that the countershaft would need changing? Brewer's lists two countershafts and two plugs. Are they different diameter, or length? They use the same spacers and thrust washers.

I don't mind machining front bearing retainers or the bellhousing, but seeing as how I'm building this thing from the ground up, I would rather just go for parts that fit.

I'd like to start looking for an iron case A body transmission, but I'd like to know what the inner diameter and depth differences are between a 307 and 308 bearing. Essentially, I'd like to change out as few parts as I can to use a standard a833 to build my gears into.