Clicking when accelerating

Hey guys I'm new to this site, after extensive searching, I finally decided on a 1970 Mustang coupe a couple weeks ago for my dad and I to work on. It runs pretty good, but I noticed a clicking/ticking sound coming from the passenger side area when driving. It seems to intensify when accelerating. We thought it was maybe an exhaust leak, so we used that as an excuse to get some hedman shortys and a new dual exhaust system. Still clicking! So now I'm thinking it may be the lifters? Does anyone have any thoughts?

Also, I am trying to decide on the right direction to go with exterior styling. I want to try to stay true to the original model, but looking through Mustangs Unlimited I don't know if I can help myself. Its a red coupe with the vinyl sports roof. I was thinking about a Boss 429 hood scoop, black front and rear spoiler, black tail light finish panel, hood stripe stencil kit etc... But in no way do I want to advertise the car as a Boss or Mach 1 when that was not the original model. This may be a dumb question, but does anyone know if those were add on options from the factory or dealer in 1970 for non Boss/Mach1 models?

Thanks guys

Thanks for the reply, yea its the same as it was before we swapped out, it doesn't do it coasting only when you press on the gas at all. I'm sure its not a rock, it seems to be coming from the back area of the engine. When you accelerate more, it clicks more rapidly as well.

Thanks for all the replies guys, gave me some good places to start. It does sound like metal on metal and I'm pretty sure it does not make the noise when accelerating in reverse. Does not sound like rattling marbles either. Thanks for the styling tips as well. I know it wouldn't be confused with Boss/Mach, I just wanted to try to add items that were listed in the catalog as being exclusive to those models and wasn't sure if they were offered as extras for others.

I'll check that too, thanks