On a budget 318 pipe size

yeah, if you're for sure going to more cubes later, and you have to replace the exhaust now; then you might as well upsize,now. And if you go with the TTI stuff, it's kinda plug-n-play. All you will lose is the downpipes.
My teener(stock LC73), ran fabulously with the 3 inch TTI duals, Dynomaxers, and their headers.With a 904loc-up and 2.76s, it pulled 2nd gear for what seemed like,forever.And such a sweet mellow tone, almost to-die-for.
I ran it that way for 2 winters.Then I got me a GearVendors, and a 3.09 o/d box. and some 3.91s,(Also tried 4.30s, but that was too much). That GV ; I used as a splitter, and it gave me 7 useable gears,(3.09-2.41-1.67-1.3-1.0-.78-.57) including a double O/D. Now that really woke the lazy-a$$ teener up! I drove it 1 winter like that and swapped in 3.55s, the next winter. Good move.
You know, when you shift the teener at 5100, and the revs only drop to a tic under 4000, and it goes on that way for what seems like forever, it sounds pretty neat; both inside the car, and from the side of the hi-way, when your son gives it a workout! I found out that LC teener pulled pretty good with a TQ and big pipes and some valve spring pressure.
Ahh, the memories