74 gearbox is a little different to a 78 in a couple of ways... the input shaft has the reverse gear cut in a different place (closer to second gear) as the 1/2 synchro slider was changed (reverse gear around the top facing second gear as opposed to the early slider having the reverse gear facing first) along with the syncromesh assembly, and the 1/2 shift fork... sort of complicated to explain without pictures, but doesn't affect the conversion process, as you chnage the entire input shaft anyway...
The trouble is that in the USA you dont get the same models that we do here in Australia, specifically the regata 85... you get the strada, but the five speed strada has inner CV's (repezza joints) not tripodes.
The regata 85 gearboxes are five speed but with tripode inner joints, and come with either 3.58:1 or 4.08:1 final drive ratio, so it make the conversion a lot simpler as far as using a complete innards to exchange.
You could get around it by using the 13 tooth pinion from a 4.08 five speed set, and then mating it to the 53 tooth crown wheel from a 4.08 four speed differential...
I've done this before too and it does work, but theroretically could lead to mesh differences as the CW&P come as a lapped pair from the factory... though I've had no problems mixing and matching as both parts were in good condition.
74 casing is a little different too, the area where the pinion bearing fits in the post 75 gearboxes is taller inside the bellhousing, thats why the clutch pressure plate (38mm tall and 45 mm tall) and flywheel (for the different heights of PP) are different, to clear this area (has a coreplug fitted) whereas the later cases don't have the coreplug , it's cast over in alloy, and the casing has less clearance.
If you wanted to do the conversion it would probably be easier to start with a later model (78 on four speed) gearbox casing, but in the US there is an abundance of x19 five speeds (unlike here in Australia) that it would be simpler to use the later model hubs/axles/flywheel and five speed gearbox complete.
The modifications to the casing to fit the extension housing are the same for all models of X19, and apply to 128 as well... I've done this to a couple of 3p's too.
SteveC