What to do to get a quicker launch...

A higher stall converter will help alot and contrary to popular belief converter you buy now that are rated at 3,000 rpm don't stall that until you mash the gas. I have a 3K converter in my Cuda and it drives like a stock converter until I floor it from a stop. Made a huge improvement in take off. Like the guy said your gears sound tall too. Sounds like you need both. If you have a 318 you don't really need an 8-3/4 though unless your running slicks. The 8-1/4 is a stout rearend and sufficient and weighs less so the 8-3/4 might actually slow you down a little. I raced a 93 Dakota for a few yrs. and it had the stock 8-1/4 in it and it held up just fine running low 14's. Ran it over 1,000 times I bet.