It sounds like the problem is that your new inserts are shorter that your old ones. MR2 ones generally are, but I don't know your exact setup. I currently have some rear sw20 inserts in the front of my Ra65 struts, which take an insert nearly the same body length as AE86 IIRC, and the MR2 front & rear inserts both have a shorter body.
Take your new insert out, lay it next to your new one, and see if the strut body lengths are the same. There is some room for tolerance when tightening the top strut not, but if the Mr2 one is significantly shorter then you have 2 options:
* shorten your struts
* get a spacer
Getting a spacer is very cheap & easy. With the tops of both strut inserts level, measure the difference in body length from the base of the mr2 insert to your old one (A). Measure the internal diameter of your struts with some calipers (B), then go to a metal shop and ask them for the the widest thick-wall piping you can get up to the width of (B), and get them to cut 2 lengths (A) long. Take an insert along with you - depending on the shape of the base of your inserts, and the width of the wall of the piping, the exact lenght you need may differ.
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