You'll need 6.5 speaker adapters. Order off of Hawgwired for those.
You don't need an amp but will want one. Your volume won't be as loud but will sound better ,Just playing off the HU.
Make sure the amp you use offers high level input, unless you want to have the HU modified.
2010 FLTRX , Stage 1, 13" Madstad, Mono tubes, SuperShox, Heritage style bars. Alpine CDE-HD149BT,Soundstream Picasso Nano Series PN4.520 , 5X7 and 6.5 Infinity Kappa speakers
If I have to explain ,you wouldn't under stand anyway.