I put together a IDF Colt 653 carbine clone using a Colt M16A1 upper, and a later A2 16 inch carbine barrel with 1X7 twist. I replaced the A2 flash hider with an A1 flash hider, and replaced the delta ring with an A1 flat ring. The front sight post is still an A2 version, and it shoots very well. My question is
if I were to replace the A2 front sight post with an A1 front sight post, will this work as far as accuracy is concerned?

I like the smaller a1 post little smaller.but will work

Will work fine.  A-1 five notches round and A-2 four notches square. Height should be the same.   I measured Colt A-1 round,  Colt A-2 and RRA A-2 square posts and they were within .003 using caliper with hold that probably wasn't perfectly square but close enough.  Height of post makes the difference.  Same = same.  Adjustment increment is simply that also.

Most any soldiers service manual or TM will break it down but would need to read one for M-16 to M-16A1 and another for A-2 for numerical specs.  Info is readily available on www with google or most any search engine but kinda mute when all you have to do is spot and adjust to your cheek weld,  eye parallax etc. for desired accuracy at desired distance.

A-1 without elevation adjustment rear will require more skill and hold over to achieve distance results generally as you can't adjust the rear rifle sight at fingertip IMO. I generally figure approximately 1" at 100 yds for A-1 sight post per click and approximately 1 1/3" for A-2 sight post.

Manual for A-1 front sight initially start at 11 clicks below flush and A-2 (adjustable elevation) generally start at flush to zero and rear off of bottom.   Since it's on an A-1 upper you would use either post starting at 11 clicks below flush and zero from there as post height should be very close or was on what I measured.  

Yes it will work.

I got my A2 front sight assembly from Brownells, shaved off the forging flash and replaced the A2 sight post with the proper A1 "five click" sight post.

I had no problems zeroing this rifle.

Building a 733 clone upper as of now.  What sight post is correct for an A1 upper? standard or f marked?  F marked is readily available. I was gonna get an 11.5" barrel with f marked installed and replace with a1 post will that work or no.  I also can get a standard post installed on a barrel from another supplier. Any help would be appreciated.

What sight post is correct for an A1 upper? standard or f marked?

Think you meant FSB but non F marked standard FSB and round sight post is correct for A-1 upper receiver regardless of barrel length within reason.

I was gonna get an 11.5" barrel with f marked installed and replace with a1 post will that work or no.

Yes,  round or square post is same height. Think you're confusing F marked FSB .040 higher with standard FSB and not the sight post but with A-1 or A-2 upper receiver standard or  F marked FSB will work fine.  

F marked FSB was instituted due to detachable carry handle flat top uppers in carbine length wouldn't zero at 25 yds without raising sight post nearly out of the front sight block. Difference is detachable carry handle is lower than standard fixed carry handle A-1 or A-2  requiring for some but not all to need a F marked FSB or install a .040 longer sight post.  

I've 2 16" LW flat top carbines using detachable carry handles with standard non F marked FSB that zero fine at 25 yds with standard sight post.  If it isn't broke don't fix it!

Best recommendation is build it and shoot it with components of choosing and rectify if a problem.  Not like they don't sell .040 taller sight posts if 25 yd zero is a problem for you.  Shooting an AR platform at only 25 yds is kind of a waste of good ammo IMO.

The A2 sight post blade is inherently more accurate than the A1 pin. When shooting iron sights being able to see the front post sharp and clear is the first fundamental in accurate shooting. The A1 being round is very light sensitive and depending on where the light is coming from will give a different sight picture. The A2 on the other hand is square which helps refine the outline of the post making it consistently more clear and less light sensitive. National Match sights are trapezoid shaped with the back of the blade wider than the front to further enhance clarity. Accurately speaking if you're going to shoot it A2 is the way to go.

ETA, the two posts are interchangeable, if you already have a zero measure the height of the current post and set the new post to that height and you should be with in a half moa.

So basically if I bought a F marked FSB installed barrel I wouldn't have issues with an A1/C7 upper sighting in the zero regardless of post type I choose to install?  I'm guessing the post will sit low or need to be cranked down more than it would using a standard non F marked FSB.