what will i really lose with 1 3/4 headers ?

It's not like you're going to take a huge HP / TQ hit. There might be a loss down low but with the Nitrous (WOT) you'll never feel it. I'd get them. What size collector? Some 1-3/4" Long Tubes come with a 2.5" collector.

Maybe I am old school, but I like to keep the header primary as close to or the same as the head is. I think that mating is the most important in an efficient exhaust... that factor and not going wild on pipe size on the rest of the system. 1.75"-2" is enough to handle any street-rod hp. When you start throwing different sizes in the exhaust gas path, it gets interrupted and you can actually hurt the goal instead of help it.

I am also a huge proponent of X-over pipes instead of H-pipes. I have used Dr. Gas x-overs since he came on the scene. They provide a much smoother transition/equalization IMHO than H-pipes. I am also in favor of exiting the rear/side with a single 2" pipe after the x-over instead of running duals out. I use 1.75" till the x-over then 2" after to a single free-flow muffler like the x-scream.

You really won't notice a drop in performance, it may actually help out. The stock cam really doesn't give the exhaust much time to escape the combustion chamber with its short duration and low lift, so those headers might help out. Either way, looks like you got a great deal and those will give you some room to grow.

lol.....most people ask these questions before they buy. you got a good deal on the parts and are in excellent condition so why worry about it. install them and run it. your not building a race engine anyways. if your worried about loosing power, swap the stock cam for something that will flow with the eddy heads or bump the nitrous up to a 125 or 150 shot will definitely make up the little h.p. loss you wont see