That shouldn't matter. If you're not running vac, just bump up the initial timing. I've run my '53 both with and without the vac hooked up with just a change to the initial setting. Runs just fine. Milage over time is a little better with it hooked up, but that's about all...
If it really needs to idle around 700, then there's a good chance it has a little more cam than stock, which could account for a little rougher idle.
Also, what does it have for a motor mount setup? I assume it has the front mount under the timing cover? if so, then it still needs the mounts that were on the sides of the bellhousing. If those were removed, then it can rock back and forth too much and could be adding to the "roughness"...
