I once owned a Saginaw 4-speed with the OD from a Saginaw 3-speed adapted to it. I don't recommend that conversion based on my experience with it, and I'd not recommend trying to adapt the DNE 4+3 from a 'vette to anything else. Frankly if that transmission is ever removed from the car while I own it something else will go back in.
I'm not sure what a T-10 with an electric overdrive is, but there was a DNE 4+1 that I believe is related to the 'vette transmission and perhaps that is what he means? I know little about that transmission except it is designed in such a way it is more swap friendly than the 4+3.
Add-on overdrives, which is really what the 4+3 is, and is what my old Saginaw was, require managing that OD. Even with the computer controlled unit in my 'vette it makes decisions about when to be in overdrive and when not to that I don't always like. I basically have to shift it manually to be in control of the combination. Around town I usually just lock the OD out and ignore it.
The shift from 1st to 2nd is problematic. Effectively its like moving the shifter the same way each time, but not knowing if it will choose 2nd or 3rd. It makes consistent smooth shifting nearly impossible.
I suspect I'd have the same problem running a Gear Vendors OD behind a manual trans too.
If I need to shift manually I'd rather just have a 5-speed or a 6-speed and have a single control (shifter) and a single process for choosing gears.
All things being equal I'd choose an automatic...
