possible tranny for v6 305E

[wc63]

i posted this in the engine and drivetrain forum but i got no response and theres just more traffic over here. so...
ive been doing some reading on tranny conversions and would like to keep the manual and possibly add an OD. i have a 3 on the tree and would have liked to keep it but freeway driving is killing me. ive read t5's are good but have also read that they're weak and tend to shred. ive also looked into the nv3500 wich requires a lot of mods as well as the nv4500 wich may fit my current bellhousing with an adapter, but requires a cross member mod and is kind of pricey. the MY6 is another option but im unsure if 1st is a granny and i havent found much info on it. ive heard that the t85n is my best bet as it is a bolt it to the 60-66 generation. but the t85n came is f*rds, if im not mistaken, so what am i missing here. i have a 1963 GMC with a 305E and would like to keep the 3 speed but am open to any suggestion that will compliment the 4:11 rear end.
thanks in advance.