360 to 904 trans ( in a 318 car)

you can buy the B&M flexplate or have the 318 flex plate balanced.

I believe you meant balance the converter? (If one can find the template/instructions for proper weight & placement.) The stock flexplate doesn't have the surface area. But the B&M flexplate is by far the best, easiest option.
If you find a 360 converter, make sure it's not a lock-up version because it won't work with a non-lock-up trans..

(Edited)