Buz, your tweaking plans sound good to me. Some people like a show car and some like to enjoy the extra spirit they personally add to the stock performance. Like Jim says, it's all for fun, just do it.
Removing the spare will make the rear end lighter and change the front to rear weight ratio. That won't make the understeer better. I have a friend who is moving his battery to the trunk. Stiffer springs and suspension rubber are worthy investments.
Matt, Regarding the Avgas, it just ain't practical. Aviation fuel is 100 octane low-lead. I ran some in my 280 shortly after purchasing it and it ran great! Until the head gasket blew. That may have blown because I found the head bolts unevenly torqued but the gas may have contributed. Also, the taxes on Avgas are WAY higher than auto gas so it will always be a buck and a half higher than 93 octane auto gas.
Ray
'68 280SL 4-spd Coupe