Way back when a friend of ours bought a brand new 250-6 Torino. We ran all big block v8's so asked him, "wont it be too slow". He responded he was used to the Model T, his other car, so the 6-cyl Torino was plenty of power.
Well I am not an expert on these, I know it is a super good basic design for economy, well designed ports, only drawback is the integral intake but for economy that doesnt matter either.
Best mod to the stocker I've heard of is to run a Carter carb unless it already has one - less troublesome and better mileage too. The link below has the super experts, this is the best 6-cyl Ford site I know of:
Fordsix Performance :: Index