Holley Contender 300-36 fits but should be ported plus I had to trim the 1206 gaskets.
My old 60's cross ram fit also but needed to be ported/gasket matched also.
You may want to read Car Craft Sept 07 issue, it has dyno results with a single plane vs dual plane. Like I said for years,both the same flow.
Did you install longer push rods ?
Luv the smell of NITRO in the morning.