tranny troubles!!!

[75??? franken-trunk]

i have a 84 short box with a 700r4 mated to a 208, it had a 305 in it and the lock up worked kinda when it wanted then it got to the point where it just went out... 1st, 2nd, 3rd, and 4th are all there but no lock up, now the truck has a really fresh 350 (355) and i want my milelage back... the tranny is pretty fresh and i never beat on really, i was thinking old converter or the silinoid any ideas???