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.

[Shadizarvan]

Thanks for your reply Lenny. I've already invested a fair bit in the TH400 and want to make the most of it. Same with the 308, both fresh rebuilds. As the van is my daily on city streets, the cam rarely sees over 2500 rpm. In future i'd like to do more highway touring otherwise I'd just use a shorter diff.