The cases are very similar, v5m21-km145, but there are serious chnges to the gears. The km145 has a thin 5th gear, and it tends to split. This is due to the gearing of the countershaft. The v5m21 changes all the mainshaft gears and the countershaft gears to allow a thicker 5th gear. I don't know if the gears will swap, but they may well do so.

The starion gears share the same problem, but the starion is lighter, and spins smaller tires, even if it handles more power. The highest stock power the 2.6T puts out is about 206-210. Jason is doing quite a bit better than that. And he tows up the Grapevine in 5th...

I have the fsm on the v5m21/km145, and note a lot of differences in the bearings and spacers and gears and snap rings - I bet swapping stuff will be a nightmare.

Look at how the tranny works. The input shaft gear spins the countershaft. Power is taken off the countershaft by locking the selected gear to the mainshaft with the synchro for that gear. In the v4m21, the countershaft is speeded up by changing the ratio of the input shaft gear and the countershaft input gear. All the other gear ratios are changed to maintain the proper gear ratios and intervals. The km145 is a 5spd modification of the km132 4psd. They grafted on 5th, and the physical constraints of shaft and counter gear ratio dictated a very small 5th gear on the output shaft. The size of the output shaft then dictated a very thin 5th gear wall. It worked ok behind 90-110hp engines, but was deemed too weak for the v6, thus the v5mt1 iron box behind that one. The starion used a lot of tricks to make the tranny survive, like limiting boost in 1st and 2nd on the 88-89.

I bet you could use all the tricks in the book, cryo and shotpeening and heat treating and still split the thin walled 5th gear under 250+hp and big tires and a heavy truck. Plus the fact that the km145 eats input shaft bearings with regularity. You might be able to have a custom 5th countergear hobbed out of unobtanium, but it would cost serious bucks.

If you must have a 5spd, it still looks like stockpiling km145 spares, a still marginal v5m21 (tho still stronger than a km145), or the options noted in my post above.