is there anyway to do it without buyin aftermarket heads (if they even make them) , and there must not be anywhere that sells flat top or domed pistons for 351m so, do i have any options?
It depends on what amount of money you want to spend. The 351M has a number of issues as originally built; all can be corrected, but depending on what you want it to do, it may cost a lot of $.
The pistons are extremely low in the bore at TDC, (i.e. low compression height) and the common cast replacement pistons are just as bad. Even the TRW forging, which is no longer available, was poor in this respect. While you could deck the block, it is usually a lot
of material to remove...and you are still stuck with whatever cast pistons you can find. You can also mill the heads, use thin head gaskets, etc., which have their own problems to correct....and you still wouldn't have much quench.
The easiest way to fix the problem, and the most common, is to convert it to a 400. If you are planning to have the crank turned, it is just as easy to buy a reground 400 crank. You then can use pistons from Tim Meyer, which are specifically designed to have a higher compression height. This solves all your compression problems in one step. http://www.tmeyerinc.com/
Other 351M issues: retarded cam timing; poor total ignition advance & curve rate; poor cylinder heads
Use a good cam & aftermarket timing chain, and a reworked or aftermarket distributor, & the next three problems are fixed.
It is a common misconception that the 351M uses 351C 2-barrel cylinder heads. They are similar...but the 351M heads have a large, restrictive, lump in the exhaust ports. This contains a cooling passage for the exhaust valve, so it can't really be smoothed or ground out.
There are a couple of options to fix this problem.
1) Use aftermarket heads. As Randy said, there are five companies making aluminum versions of the 351C heads, in various chamber & port configurations. They are a direct bolt-on (putting aside the intake manifold issue for the moment).
2) Use 351C 2V heads. Again, a direct bolt-on. They have large open chambers which are a topic of discussion (good & bad), but they are common, cheap, & seem to work well enough.
3) Use the vaunted "Aussie" heads- which were made for the 302C Australian Cleveland. They have the smaller 2V ports, with the closed chamber design of the 4V heads...a nice combination. However, they have small chambers and it is difficult to keep the compression ratio on a 400 low enough to be streetable without a deal of thought.
4) Use 351-4V heads, either open chamber or CC.
There are not a whole lot of intake manifold options for the 351M/400, as such. Everything I know of is made for the 2V heads. It is possible, though, to use a 351C 4V intake (whatever brand), with 4V heads on a 351M/400, with adapters from Price Motorsport. They aren't especially cheap, but then what is, nowadays. You can also get adapters to use a 351C 2V intake with 351C 2V heads on the 351M/400 block.
It all comes down to desired power vs. budget. It is possible to build a respectable 400 for a reasonable amount of $, so long as your expectations are reasonable as well.