<<< Does that mean that if it had the blocker style synchros I should buy it since the gear set would be identical to mine except for the 4th gear? >>>
I can't say because I don't know that much about 128 ratios. I can only say that I know the later 128s used the blocker rings on 1st and 2nd. I am pretty sure that the 128SL used the X1/9 gearset entirely. But it has been years since I built an SL unit and can't say for sure.
<<< Or does the different ratio make everything different? >>>
If any single ratio is different then the input shaft dictates which ratio set you can use with it. So if only 4th gear is different then you can't use the input shaft or 4th gear but the others can be. That is assuming it uses the blocker rings on 1st/2nd.