FMX troubles…………?

Befor i did the full engine reco on my 351w 1969 vert, the trans was working fine, doing all the right things, on time, all good.
Since putting the engine back in, it is sometimes flaring like its out of gear momentarily and holding on to low gear a bit to long, and falling in to neutral wen stopped.
I checked the vacuum hoses as best i can with out taking things apart.
Any good ideas any one ?

First, start the engine, shift the trans through 1, 2, D, R and put it in Park. Leave the engine idling. Pull the dipstick, wipe it, restick it and see if the level is low. If it’s full, is there any chance you didn’t have the torque convertor engaged all the way during engine installation? Some of those symptoms are of low pressure. The convertor hub could have been on top of the pump gear causing a bit damage before the hub popped into place to the inner pump gear. I hope not, but it doesn’t add up to vacuum hoses.

could it just be a bit gummed up inside the valve body from sitting a long time ?
im reluctant to try a long drive if its going to fail ?

If it worked fine before the engine swap and exhibits problems after, it seems as if something went astray. How long was the engine out of the car? Unlikely anything is gummed up, unless it was more than a year and was subjected to moisture. There could be valves sticking from suspended metallic, but the metallic is an indicator of condition that may suggest the trans is in need of rebuild. If you’ve verified all the vacuum lines are correct and there are no other installation issues. Next step would be to drop the pan and check for debris. If the trans is original or was rebuilt more than 20 years ago, it is probably due to be freshened up considering the symptoms.

i put 1 x litre of trans oil in it and drove it,
seems to have been the simple solution.
Couldn’t get an accurate reading on the dip stick,
possibly not the correct stick ?