I know that because all of the gears are on the countershaft, you cant mix a 151 first gear on to any other countershaft.  However after talking to our builder, the 5th gear comes off the countershaft and output shaft.  So if you got a whole turbo gearset, and then you take 5th gear off both the countershaft and the output, you can put a R154 5th gear on to that.  Hope that helps.