Gearvendors overdrive

[Shadizarvan]

Hi all. I have a HJ van with 308 and TH400 with a 3.08 diff. The van is my daily and I am considering acquiring a Gearvendors overdrive unit https://gearvendors.com/index.html/ so I can drop the diff ratio to3.9 for a bit of poke off the line whilst still retaining highway manners. Does anyoneknow if the tunnel has to be clearanced for such a modification. I saw a video of an install into a Corvette where some bracketry had to be trimmed. Any tips or advice would also be appreciated.
Cheers

[axistr]

Probably be cheaper to fit a Tremec TKX 5 speed or T56 Magnum gearbox or VF 6 speed, and a more engaged drive. 3.08 is a good ratio for manuals however 3.55 would give you a touch more go off the lights. TKX will fit in the tunnel without any real mods to the car. Almost a clean swap from an aussie 4 speed. Can be done reasonably cheap if you buy right and have a bit of mechanical knowledge. If you have to have an auto then I'd look at some of the later model autos like the 4L60 That can be controlled by a aftermarket shift controllers. I have 3.9 gears with a Tremec 6 speed in one of my Torana's (Holden 304 V8), I reckon it would actually perform better with 3.55 or 3.36 ratio. Use some of the V8 torque for a daily driver.