First post whooooohooo!

I have a 98 f150 4.2v6 with a new factory crate motor installed and i just replaced the trans with a rebuilt including all the hard parts. For some reasone it is still very sluggish at take off and i am only getting 10 mpg which blows. When at a stop and i press on the gas it barly moves then it will gradually get faster and faster. I took it back to the shop and they said that there is nothing wrong with the trans. The shop said it might be a clogged cat but they havent ran a diagnostic on it yet, do any of you have any thoughts on what it might be before i spend even more money on it?

Thanks!
Jeff