For what it's worth, years ago I had my CB350 swinger lengthened by 4" or so... At the time I didn't know any better, so the shock mounts all stayed where Honda wanted them. Dropped the ass an inch or so. Maybe more. Thought it was super cool. Rode it for awhile (maybe 100 miles max) before I grenade the top end.
Fast forward to now.
I've had several bikes since then. From hardtail chops, to all stock early 80s first Gen go fast stuff. Current daily is a '77 GS750 (all stock). Having, over the past so many years, ridden this multitude of bikes (plus several others) I've learned a few things. I've also been lucky enough to glean a good bit of knowledge from this site and the friends in the hobby that I've been lucky enough to meet. I've also made several mistakes in a number of builds, which as we all know is the best way to learn a lesson. Why am I rambling on about this you ask?
Because I want you to understand that what follows isn't some random Internet hate. I'm miles away from being as talented or skilled as many here but, I'm pretty good at learning what works and what doesn't (even if it sometimes takes a minute to sink in).
Don't lengthen this swinger. You really don't want to, and here's why.
By lengthening this swinger, and not compensating for the massive change in rear suspension function you've made something infinitely worse than stock.
You mention lowering the bike by 2.5" as a goal. This will also effectively ruin this motorcycles ability to function correctly.
By adding 4" in wheelbase length, you will need to also lengthen the chain allowing for a ton of slop. There are ways to fix this but now you're complicating a simple system with no benefit. Not to mention the fact that during rough conditions, the chain will likely rub the swinger (ask me how I know).
You're making a massive structural change to the swinger, which in stock trim is spindly and flexes, effectively weakening it. Add that to that the new shock angle and it makes for a terribly squirrelly ass end (again, ask how I know).
I think know you get what I'm after here... I'll quit being a stick in the mud now.
Just make sure you COMPLETELY think these things through. There's a lot of wasted time, money and effort that can be avoided by a few hours in the garage, a cold beer and an extra set of eyes (and no chop saws
).