Is the stock flexplate on a 307 a 168 tooth or a 153 tooth? I am replacing the 307 with a 350 and I need to order a flexplate for the 350. I just want to make sure that I order something that is compatible. Thanks.

I have never seen a 307 with a 168 tooth flexplate (or a 168 tooth manual flywheel for that matter). BUUUUUUUUUUUUUUUUT, it certainly will bolt up to a 307 just fine. The 307 was a LOW-perf, 2bl engine and generally got the smaller 153 tooth flywheel/flexplate. The super easy, double quick way to tell is to look at your starter. Yes, look at the starter. If the bolt pattern on the starter where it bolts to the bottom of the block is a straight across bolt pattern, then your flexplate is the smaller 153 tooth version. The larger, 168 tooth flexplates/flywheels must use a starter with a staggered bolt pattern.
Does that help?

Oh ya, yes, that means if you put a 168 tooth flexplate on the 350, then no, you cannot use the starter from the 307!!! You'll have to get a starter with staggered bolt pattern! I'd suggest you get a starter with the stronger cast iron nose instead of an alum nose. And, in order to get the cast iron nose on a starter (trust me, the kids at the parts stores won't know this), you MUST order a starter for a Chevelle with a BB (396, 402, 454) AND A MANUAL TRANSMISSION (4SP)!!! If you SHOULD decide to get the starter with the iron nose, when the parts guy hands you the box, OPEN IT AND BE SURE THE STARTER NOSE IS IRON AND NOT ALUM.
All of the above holds true for a factory type GM DELCO type starter (new or rebuilt). It may not necessarily be the case for an aftermarket or mini starter.

thanks tom, is there any difference between the 168 vs the 153 tooth (besides the obvious difference in the number of teeth). I guess what i am aking is if i put the 153 tooth flexplate on on the 350 ( so I can reuse the the same starter as on the 307) will there be any problems with durabilty?

Nope, just unbolt the 153 tooth flexplate from the crankshaft of the 307, bolt it up to the 350 and go on down the road! That's it!
Of course the other difference between a 153 tooth and a 168 tooth flexplate/flywheel is their diameter. The 153 is 13in and the 168 is 14in. That's why you have to switch starters is because the staggered bolt pattern starter is further away from the engine to accomodate the larger diameter flexplate.
By the way, there are ONLY TWO sizes of flexplates/flywheels:
14in diameter with 168 teeth
13in diameter with 153 teeth.
Up until 62, there was ONLY ONE size, 168 teeth. Then, in 63, the smaller 153 teeth flywheel was introduced and it was GENERALLY used on the low perf engines and the bigger flywheel was used on higher perf or big displacement engines. ALLLLLLLLLLLLLLL the way up to 85, ALL crankshafts and flywheels/flexplates share the same bolt pattern, so they are totally interchangeable.
Now, if you want to install an 86-later engine in an early car (say for example a 1990 350 into a 55 Chevy), then you have to use an 86-later flywheel/flexplate. But, that flywheel will still mate with the torque converter of an earlier tranny.
You got all that down now? There will be a test. A passing grade will be 98 or higher. Anything lower than a 98 will be a failing grade.

Okay correct me if Im wrong but, I thought the 153 tooth flexplate was for a power glide application and the 168 flexplate was for the turbo 350 and turbo 400 application. 168 tooth 14 in. 153 tooth 12 3/4 in.diameter. Jim

Jim,
Yes, basically, that is exactly correct. But, the main issue here was if the smaller flexplate/starter combo from the 307 would work with the 350.
When my son broke the 307 in his 68, we unbolted the PG/flexplate from the 307, went down to O'Reillys, bought a 350 long block, transfered all the parts/hardware from the 307 onto the 350, buttoned it all up to the PG/flexplate and he is going again. ZERO modifications whatsoever, except I installed a 68 Q-jet/manifold on the 350 instead of the 2bl from the 307.
Oh heck yes, then he used the new 350 to rip out the 3.36 10 bolt non-posi rear!!!!! So, then we installed the 3.08 10 bolt that was originally in my 70 conv into his 68 (I told him the next one is on him and don't ask me for help or $$$$$). Although, I do have a 10 bolt 2.56 from my other son's 71 that I guess we could use. If so, then I am out of spare rears!

I learned this the hard way myself, when I originally swapped the 307 PG out of my camaro to a 350-t350. It was all the little things that adds up. flex plate starter, floor shifter. At least now I keep a lot of the odd ball stuff, so my last two projects went pretty smooth, Jim

My original 350, L-48 starter bit the dust so I took it off and went to the local Parts Store and told the parts person I wanted a h-d one AND lo and behold I got a 427/454 h-d rebuilt starter!

pdq67