[rocknss]

I would swap in a 700-R4 so you have overdrive, that's what I did to my 69 Burb that had a 4 on the floor too. You can find them easy, just get a 88 or newer one.

To do the swap to an auto you need these items besides the trans.

auto flex plate
auto starter
auto trans cross member
radiator with auto trans cooler
transmission TV cable (after I completed swap took Burb to trans shop to have them adjust TV cable correctly)
swap to auto steering column or do what I did and use a floor auto shifter
will need to shorten drive shaft (have it balanced too)
install torque converter lockup if swapping to 700-R4 ( easy to install)

I don't think a TH350 swap would be easier than a 700-R4 because you still need all items but the torque converter lockup and depending on your engine you might want the OD.