A longer wheelbase makes for more high speed stability and sluggish turning. Since both woods riding and MX require responsive turning my recommendation would be also to stick with the stock swingarm length or extend it only a little if you need to shift the weight bias.
For what it's worth, I think some of the factory Honda riders used to even run with SHORTENED swingers in some cases.
CURRENT RIDES:
1988 Tri-Z 350 Twin
1985 1/2 Tri-Z
1984 IT490
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