A cap will help.
Check Ikesound.com for that.. they've got round caps that are even less than $40 quite often. (with caps, brand name matters not... just be sure you buy a round electrolytic cap.. the square kind are bad, high ESL)
I definitely would recommend upgrading the "magic 3' though.
Buy some 4 gauge wire, maybe 10 feet of it... and at least six 4 gauge ring terminals.
The magic 3:
1) wire going from your battery - to the chassis ground. Upgrade the wire, and also sand the paint away so it's actually a metal-on-metal connection (paint is an insulator). Paint over the connection, or grease it so it doesn't rust.
2) wire going from your alternator to your battery +. Upgrade the wire to the 4 gauge.
3) wire going from your engine to the chassis ground. Upgrade the wire, and also sand the paint away so it's actually a metal-on-metal connection. Paint or grease it too... this wire is essentially your alternator's ground wire, since the alternator is chassis grounded, is bolted to your engine, and this wire grounds the engine.
Good luck!