I've used a small chain across the torsion bars to hold up the front of the transmission. Might be able to do the same for the back of the engine. Come to think of it I've used it for the engine too.
Same situation as yours except column shift. Changed rag joint out for Flaming River U joint. Ebay 96 Dakota starter. No dents needed. Even used studs for the outside header bolts. Installed with provided instructions.
I would think a stroked 340 with an xe268 cam would seem small and pump out the torque all the way through the RPMs. Had to do a lot of tuning to get my 318 to run right with that cam with a stock converter.
Probably not stroked.
This is what I run on my cars, does great. I especially like the rev limiter, probably saved my engine more than once.
https://4secondsflat.com/J685HRRCoilsprings.jpg
I would suggest shims against the hold-downs to keep the rockers from eating into the aluminum. Just mounting and turning the engine produced scratches in my case and I would think it would be the same with the springs pressing the rockers against them. I had the hold-downs machined so I could...
If you have all the pulleys, brackets, accessories, and timing cover from your 318, I would do that. Just need two piece balancer from 5.9 Magnum. Good luck!