I think I remember that your machine takes system 1798. I think it's engraved on the bed slide or needle plate. Is that right?
Here is the Ismacs chart that shows the dimensions for round shanks:
http://www.ismacs.net/needle_and_sha...und-shank.html
1798 is 9th row from top.
Two different lengths of needle can work if the distance from the top of the needle to the top of the eye is correct, and the needle doesn't hit the hook or bobbin case. But be careful because I've read that the bobbin case can be grazed without making a sound or vibration.
Was the 70 needle a different length than the 90 needle? If not, you could try it on a lightweight cotton. Sometimes on thin knits or slippery fabrics the thread doesn't make a proper loop for the hook to catch, but usually that results in skipped stitches.
Oh- Another thing -- the engraving on your slide plate might be actual size -- compare your needles with it.