Mike,
I just did a swap behind my 305V-6 this past summer and have a full tutorial written I also posted it on the BB as well. You can find it here
https://6066gmcclub.com/showthread.php?t=50844
I had an old and worn out 4 speed Hydramatic, the rebuild cost were $3200+ and I still would NOT have overdrive. I put in a GM 700R4 (non-electronic 4 speed with overdrive). The 305 REALY LOVES THIS! I attached a p[ic of the truck doing 80 MPH and only turning 1850 rpm (I have 3.23 danna 44 rear gears).
To go forward, you will need to do 1 of 2 options. Find an factory original automatic adaptor setup (pictures attached), you will need the flywheel, spacers, flex plate and adaptor ring (they are out there , but you got to really search) or you can simply buy one from Bendtsend Transmission Adaptor for approx. $1200. I found my factory unit for $400, plus shipping.
The swap is simply and NO ELECTRONICS. However, you will need to purchase the Bowtie Overdrive kickdown kit ($200) and Carb adaptor plate/Wire accelerator set up, you will no longer be able to use the bell crank pedal set up as the adaptor plate will interfere with the movement.
Its a super simple swap and I did it myself with a friends help with NO experience. You will most probably want to work from lift to make it easy as we did.