What year is your car?  1970?  Virtually any of the major mfgs' headers will work.  JBA's are one of the best.

I have Heddman shorties on  my '66 w/70 351w.  Much better than long tubes  Long tubes give much less room around sensitive things like starters, steering, etc.  Heat is a problem too.  You might have problems with the long tubes hanging too low and scraping the ground on humps.  particularly if it is lowered.  Spark plug accessability is an issue too.  The guys here can help you out there.  And with shorties there is very little hp loss until you rev the engine real high.

Part numbers and vehicle info would be great. Auto,5 spd, air,ps, pb stock steering or rack and pinion? are they for a certain year vehicle only or 65-73? Just a few questions that may apply.
​Sorry guys but there are so many variables that this could be a what do you have and what fits topic.
 

An X pipe has smoother flow, and in theory helps with power, but my guess is that on a typical street engine there's little if any observable difference. 

That's true too.  If you want rumpity just forgo the crossover.  My car doesn't have one and it sounds like its got a bit of a cam to it even though its just a stock A code 289. 

I have an H pipe on it now and it is holding the system stable. I've read that an X pipe makes more power but with my set up I think that is negligible except that I like the idea of removing the "rumpity". IMO the "rum pity" is a Chevy thing. I just don't know if the X pipe will work in a convertable because the chassis stiffener plate might be in the way.