The input shaft on the smaller engine T5 is to long and would need to be replaced. I know the gear ratios are different as well. There may also be a strength issue as well, I'm not 100% sure on that one though.
Non-V8 T-5's are significantly weaker. The gear ratios are not well suited to V-8's either. As far as replacing internal parts to convert it to a v-8 spec transmission, I cannot speak to if it is possible, but I can tell you it is cost prohibative. At one point I thought I would rebuild a V-8 T-5 that may have needed a new main shaft. The new main shaft and gears were $500. This did not include bearing or the general rebuild kit. After thinking about it, I returned the gears and shaft and had them send me a new T-5z.
Stock, rebuilt units can be had for $800 or so and new, heavy duty units are between $1000 and $1100. Not cheap, but neither are the hard parts for these units.