Ok, first of all, my truck is a 1988 F-150 4x4 with a 5.0 and a 4 speed.

#1. When going down the road if i down shift, or let off the gas in 3rd or 2nd gear, there's a "popping" sound and occasional "bucking".

#2. I already have a K&N CAI, shorty headers, and true duals. I'm planning on getting new plugs, new MSD spark plug wires, a MSD coil, and a MSD cap and rotor.
a. What plugs should i get?
b. Should I get 8.5mm or 9mm?
c. what would be some other reasonably priced parts for a nice combination of MPG and Power?

 

personally on pretty much a stock engine I would save the money
that you would spend on the MSD and go with normal replacement parts.

I think stock wires will do you your not running a CD ignition and you dont need a CD ignition cause you have stock compression ratio.

 

I'd agree with Dom on this one -
New plugs should be the motorcraft stock plugs gapped per the emissions sticker under the hood.
Wire size is not as important as quality - get good wires, and let your wallet decide if you want bigger (you probably won't notice a difference with bigger ones).
The coil, cap, and rotor - you'll benefit from replacing these, but you probably won't see a performance difference versus stock new parts.

What sort of 'popping' sound? Is it through your exhaust, or something in your drivetrain? Might be worth giving your U-joints a good check.

 

thanks!

its not exhaust, and i just had both front u-joints replaced. but i'll check the back ones again.
its hard to describe. just like popping. not rapid tho.