Mick, I missed this as I was away in the Corvette, in the Alps! Glad the additional fluid sorted the issue, and interesting that the low level caused the symptoms in did, especially the right turn thing!
After 1080 miles, much of it going up and down Cols, I think the old '78 is due a fluid and filter change over the winter! Such a "fun" job but, slightly less messy now that I know to pump out the fluid before releasing the trans sump bolts!
Its not so difficult if you release the bolts slowly from one end to another so the pan gradually drops down at one end first.
To collect the old fluid, I use one of those large plastic 'under bed storage' boxes with the removable lid. Big enough to catch all the fluid as it begins to run out.
Just for advice, when I did the 700R4 on my '87, I bought an AC Delco "professional" filter kit (Chinese made) but there's no way the new seal would press in attached to the filter pipe (tried for ages but it wouldn't fit and by this time the base of the filter itself was being pushed in) and I couldn't reuse the old seal as I had damaged it digging it out.
A contact to Rockauto and they apologised and refunded me, saying just bin the AC unit.
So, I bought an ATP 3rd hand kit off Rockauto and it fitted a treat! 3rd hand means that the gasket has two holes each side undersized to retain four of the screws so you can refit the pan by yourself, torquing the bolts to 10lbsft for a perfect, non leaking quick and simple job with minimal mess..
So simple, I decided to do the 4L60E on Michelles S10 truck too with another ATP filter/gasket 3rd hand product, again no problem and this time I added a bottle of Lucasoil Stop Slip as we had a problem with the D and OD clutch pack slipping.. Glad to say it doesn't slip anymore!