I am thinking of changing my old holley to a new edelbrock and I my question is/ what is the difference between the street carb and the marine and can a street version be converted to a marine carb easily

I think you're making a mistake first of all. I can rebuild a Holley with my eyes closed, but when my Edelbrock needed a rebuild, there were so many parts, I took it to a shop to have it done.

there is nothing wrong with the carb but it probably could use a good freshn up. any suggestions

I still want to know the big difference between a marine carb and a street carb in the edelbrock carbs

The USCG sets the standards. So it has to not be able to leak past the throttle shaft, and the vents have to go back into the carb.

so here it is I have a very close to new edelbrock 650 that i have on a 77 big block gmc that i am selling and i hate to just let it go . after all it cos about 4 bills . so can i convert it easily or am i pissing up a rope.

If its like a Holley, it can't be converted 100% to "approved" marine because of the seals on the throttle shaft. I don't know how Edelbrock makes them marine legal. You may be able to buy the needed parts to make it legal from them.

Contact edelbrock. If they can sell you conversion parts, they probably will. If it cant be converted to legitamate USCG approved, it's not worth trying.
Gary