these engines won't bolt onto a DB because the stock 2.8 has a smaller bolt pattern. you can either drill/slot new holes or buy an engine mount plate from cjtmini (member here). he also sells complete kits with everything you need to get going with a 6.5 (engine and clutch included). i'm running his plate on my DB and it is worth the money IMO.
if you choose to drill new holes, be sure that the clutch sprocket and rear sprocket are lined up. if they're off, you'll end up throwing the chain which is a pain in the ass to deal with.
you will also need to get a new clutch. the 2.8 has a 5/8" shaft and the 6.5 has a 3/4". a Max Torque or Comet clutch will work fine. you'll need one for #35 chain.
other than that, you just need to hook up the throttle and maybe remove a link or two from the chain. it's a good idea to get a chain tool for removing links. if you use it once, it's worth it. check out
BMI Karts and Supplies - Home for one, they have them for about $12. just be sure to get the right one for your size chain.
do a search in the DB section for info on hooking up the throttle. there have been several threads showing different ways of doing it.
