How stock is the 455? If it is mild, the stock 2004R will live with a few key hard parts like the stator and forward drum. Add a 10 vane pump, big shift kit with bigger boost valves, billet servo and quality convertor, clutch,bands and your there. The TH350 is no prize stock, I have seen two smoked by a stock 75 and 76 Olds 350. We are talking 165hp/265 tq, pretty poor that they died. I know someone who put stock TH350's behind a stock 70's 454 in a 79 Camaro swap. He blew 3 or 4 in short order. He added a shift kit to the last one and sold the car. The 4L80E is the best of both worlds but needs an adapter and controller or manual valve body. Be aware that you will eventually explode your spider gears in the 7.5" rear too.