First modifications...
This will reduce the body flex and preserve the integrity of your car. Basically, reduce the chance of squeeks, leaks, and the like. Now, if you do not intend on keeping the car, then I would suggest headers next, b/c most modifications will be benefited by the increased exhaust output. IMO, that is what I would do!
id personally do trans and rear end work, have some head work done, put it on a tire and spray the hell out of it but thats just me.

