That transaxle, isn't much stronger that a std VW bug. The 550 Spyder was only 1500CC. The Bug transaxle can be upgraded to hold 200hp, with all Hewland innards and would be stronger than the 550. That's what is in my car, pictured below.
A ZF transaxle, like used in the original 289, GT40 would be bare minimum strength for a FE and you'd have to be careful. But, the Hewland, 4 speed, LG500, is the one that will work. They are expensive and hard to find. The LG stands for Large Gear Box, the 500 for torque.