I have a 1986 Chevy c10 original 305 700r changed to a 350 still have the original trans and it doesn't like the extra hp one bit it'* gone through a converter or two by now and it'* starting to slip In drive so I was wondering if anyone has any advice on swapping to a 4 spd auto or 5 spd auto I talked to a very angry transmission guy who didn't want to help in the first place and he said it would be super expensive and near impossible I'm not sure if what he was saying is true or he just didn't want to help but I do want the extra mpg so can anyone give me some pointers

you can build the 700 to handle a mild 350 no problem, much easier than trying to swap anything. just have to find a better builder, look for a performance trans place, like racing transmissions. if you are near florida try going on full size chevy forum and search for transbuilderguy.

You can put a electronic transmission like a 4L80e into your truck but the controller is expensive to run the thing. As Justin said you and build a 700r4 to handle up to 400hp with a newer front pump and good clutches. Here is a link for the electronic controller.
Chevrolet Performance Automatic Transmission Controllers 19212657 - Free Shipping on Orders Over $99 at Summit Racing

Curious if you are hauling a lot of weight with this truck or something? The THM350 isn't the most super stout transmission out there, but should be able to handle a stock 305'* 140HP no problem.