I have had basically the same set up that you have, transgo shift kit , 4.10s , stock long block and it pulled very hard to 5600. Thats with most bolt ons , 1.7 roller rockers and a worked upper and lower intake though. I did have a similar problem to yours in my 92 hatch and it ended up being a vacuum leak on the lower intake. Could be a whole host of things unfortunately. Good Luck, Nice Car and Welcome to Stangnet!