215/75 15 muds, bit of trimmin on the front bumper, maybe some off set wheels, or decent spacers, and should be all you would need
My friend had his old 410 with the 7" white 8 spoke, and Diamond backs (215/80/15's) ran on standard suspension, got good flex, worked well off road, and the tyre size wasn't so bad that it killed the on road performance
or. save up and buy some springs, one of my moto's now i am older, is, do it once, do it right or else you will have to do it all over again
pic for refrence
with regards to shackles, although i hate them, if you extend the bump stops, shackles should not kill the springs, the springs only die as they bend beyond flat, my badly done SPOA is testerment to that, no bump stops at all, and springs are S shaped.