Figure out one thing at a time. Start by finding out what the rear axle gear is. Pretty easy to do.
Watch this video. If you have a power glide that works properly, then you might not be any happier with a TH350 automatic. Both have the same 1:1 final drive ratio.
A good cruising rear axle gear is a 3.08 or 3.42. If you decide on an overdrive transmission, then a common rear axle gear of 3.73 works well.
But I’d definitely start by figuring out what your rear axle gear is.
