how much difference is it realy goin to make hp wise in a lt1 using 1 5/8 rather than 1 3/4?

Stock LT1, probably not much difference.

forgot to mention its the motor in my sig. i want to get into 11's with nitrous but i want to run 12s without and im thinking of how much a loss the 1 5/8 is gonna be compared to a 1 3/4.

I'm running Hedman 1-5/8" headers and I'm at 12.8 @ 110 with almost a full LT4 conversion with Hotcam. 1-5/8" will be fine, but you'll still make more with 1-3/4".

Just to give you an idea; almost every set of headers made for LT1 4th's come with 1 3/4" primaries.

how bout a 1 5/8th and a cutout... or is the damage done already after the headers???

cutout definitely helps, but the only way you're going to get all the power out of it is with 1-3/4" primaries.

with that 306 youd be foolish to go with anything other then the 1 3/4..also remember that the exhaust port for LT1/4 heads are d-ports...not the traditional oval port as in SBC's

ok ill will probraly go with a 1 3/4. any recomendations?? long tube or shortys?? brands that are good but below 500$ i found a nice set of slp shortys but they sure arent cheap. if i could get away with a set of long tubes i would rather do that but i realy dont like the idea of them hanging so low and i have yet to find a set.

forgot to add i need them emmision legal. i meant to say i could find long tubes but non with air tubes.

Don't think you'll find emission legal longtubes. I think maybe only hooker super comps will be the only ones.

is there any way i can drill and add smog tubes? i just need to pass visual inspection.

Get the SLP shorty's, you can get them for 400 easy and 360 or so if you try, 1 3/4 non air, the air setup is different on an LT1, then add the fittings like the SLP LT1 set, take a look at their site for pics of the LT1 set and you'll see what you need to add.