Rather than pipe nipples standing on end, I'd be more inclined to try making temporary spacers out of some short strips of 3/4" x 2" lumber (lengthwise under the seat track & painted black) with a hole for the seat frame bolt drilled thru it.
If 3/4" isn't enough height try some 4 to 6 inch lengths of 2x4, laid flat with a hole for the seat bolt.
Spreading the stress & weight out with larger spacers like these will help keep the longer seat bolts from folding over in case of hard cornering or an accident. Don't forget to use large thick washers under the floor too.
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