I think in his case it is a matter of finding a dead car for a live motor... what he has is a 3.5 running, and happened to find a dead 3.3 car to receive it.
He seems to have both cars there...
I don't know - I don't think the 3.3 and 3.5 driven the same way have much of a real fuel mileage difference. I have NOT ridden in a prepped 3.3 car - but I can tell you bare stock 3.3 and 3.5 - night and day difference in zip... Maybe a prepped 3.3 like you or Dave have (had) would be a bit different, but then, I think you'd have to prep a 3.5 the same way for an apples to apples comparison.
Oh mine is is not peppy. Makes noise for sure but really stock besides a little experimental porting. I am sure I am barely around original 160hp ratings with 207K on it. However I will say it still hows plenty of TQ and when used with autostick in the power band, which is, all of it almost lol seems to move fine. However I can drive out to the Dragon (two hours mostly highway 70+) beat the snot out of it, staying in second and pretty much keeping the rpms needle from dropping below 4k for several hours, drive back and still get 26+ mpg.