The easy OD tranny for you would be a NV3500 from any 2wd full size truck or a 4.3 S10. get the bell & clutch to go with it and you should bolt up to SBC
A S10 T5 from approx. '89 or up is worldclass and has almost same torque rating as NV3500. But here you need either a car bellhousing plus a spacer or a truck bellhousing plus a spacer or a jeep input shaft and no spacer with either bell. A board member wrote the bible on T5 swaps
https://lugnutz65chevystepside.weebl...info-page.html
I have a t5 in my 1952/250 and a '95 sonoma/4.8LS they are good transmissions and can still be had for cheap if you are patient.
nv4500 would be more HD truck like, but overkill for a stockish 350 and they are much pricier around here.
These OD trans will help, but as dsraven says you probably need some different rear gears to make it highway capable in today's traffic. depending on cargo expectations and driving style some upgraded front brakes may also be needed