The shock cords that ship with the rocket are cloth and elastic and so have a lot of give in them. If you were to switch to nylon or kevlar, you need the extra length to absorb the same energy and thus why they're so much longer. Nylon does stretch some but kevlar doesn't. Personally, I'd go with the 18ft section of 9/16ths tubular nylon. It'll be more than enough and is still fairly cheap. Only reason to go with kevlar is for the flame resistance and tubular nylon will be good for dozens of flights at least. I've had PML kits with over 30 flights and the tubular nylon was scorched but still viable.
If you're going to modify the motor mount to accept motors longer than the standard AT 29/40-120, then you should make sure to use epoxy to attach the fins. Plastic fins wouldn't accept fiberglass well so doing a tip-to-tip wouldn't be advisable. That would also mean you'd have to do something else besides the baffle system that AT uses in that kit. If you're going to go through all that, I would drop the baffle and motor hook and use an Aeropack retainer with the separate baffle further up the body. LOC and Apogee both make good baffles that should fit that tube and provide decent shock cord mount points but you could scratch build one just as good if you wanted.
What I would do:
- swap the elastic cord for 18ft or so of 9/16th tubular nylon
- swap the 2 parachutes (but keep them!! they're really good and lightweight) for a single 48" with a spill hole. I have tons from LOC and PML kits over the years that I move between rockets.
- swap the motor hook for an Aeropack to accept large H motors or baby I motors
- swap the baffle for either a nomex blanket or 3rd party baffle. If you go with nomex, you'll have to figure out a shock cord attachment. Epoxying the shock cord directly to the MMT through the forward ring is my normal way.
- use epoxy to build the fin can. It'll be plenty strong as long as you don't land fast on a hard surface. If that is a worry, upgrade to a slightly larger parachute. I keep stock from 18in to 96in just for these kinds of reasons.
- secondary option - add an electronics bay and go dual deploy and ignore the motor ejection. A 2.6" body tube isn't where I'd want to learn dual deploy but I add it to any rocket 3" and up so its not that hard to put on a 2.6" With dual deploy, I'd use the supplied parachutes and swap to 10' and 15' sections of tubular nylon for the shock cords.