Research how well your current mufflers flow, to see if they match the pipe dimensions. The book How to Build Horsepower, second edition, has a ton of info on exhaust systems and getting the most out of them. If your current mufflers are restrictive, you can figure out roughly what a change is worth based on how good the rest of your system is, and how much the mufflers are choking the system. To get the flow increase you want you might need to use reducers to go bigger on the entry/exit of the muffler you want to go to to get extra flow.
My next move would be a GOOD converter, after that, posi, plus suspension tricks to get some weight transfer. since you want a decent lockup at cruise based on your comments. I am not sure how much can be done to get the manners you want plus a good stall increase, but a conversation with a couple of top converter builders ought to give you something to go on. A good converter is worth more ET than a gear change, and a stroker would really pick up the 60 ft with just a better converter and posi plus good tires, if you really want to turn up the wick.