Thinking of changing from edelbrock performer to a comp XE262, in 78 camaro (3650 lbs)355 have standard converter, edelbrock performer manifold, s/side portted heads, ( 9.55:1cr ) flowtech headers, holley 600vac/sec, but rear is a 2.41 at present willing to change to a 3.08, no higher as want sensible crusing rpm's at 75,80.
c/cams states a XE268 is largest for standard converter, but idle's at 15.5", whereas a XE262 makes 17.5".

I'd go with the 268, it will still have a smooth idle, and should pull a minimum
15.5 in of vac. (I think thats idling at 500-600rpm), if you idle it up to a normal
700rpm it should pull plenty of vac. - a cam this mild should pull around
20 in. - plenty for accessories. Plus I think UDHarold designed the 268.
With a 308 gear you should be around 2500rpm @ 80mph.

UDHarold designed the 268HE, the High Energy cam, not the XE268. The XE268 is much more agressive, and at least in my case, very noisy.

Of the two i have to say that the 268HE would probably be the better choice, especially with the high gears you plan on using.Oh yeah, get the 3.08's, you and your engine will be happier.