A ford spacer might work...
I have icing issues too,on an edelbrock 600 cfm performer--they are lousy carbs in the winter,when I had it on my 250 six in my 79 C10 it would frost up like a snow cone and cause it to die every time you let off the gas,got very annoying to drive,and dangerous too,at times...its now on a 305(250 RIP

) and I can hook up the "hot air pipe" thing,but it still ices up some in artic weather...
I was going to get a carb spacer I saw at a boneyard on a 390 ford truck manifold,it was a 4 bbl spacer with 2 heater hoses on it,just like the "clifford" one described in the last post--but I was in a hurry and could not get it that day,and now the truck is gone,probably went in the crusher

--so much for that idea,unless I find another one someday--I saw this one in a ford pickup,I'd say a 74-77 vintage,with a 360 or 390 "FE" motor--they used ford-autolite Holley style 4 bbl carbs,so the base should be the same as your Holley carb..
I would not underestimate the importance of having heat to the carb base--not only does it make it safer to drive by preventing stalling at a bad intersection,the motor itself suffers when the mixture is too cold,it makes it run rich and form lots of sooty carbon--I think this is what killed the 250 six I had the 4 bbl on for 2 years,driving it in the cold weather allowed enough carbon to form that it cracked the #6 piston's top,and a big chunk broke out of it,exposing the top rings!--the motor had only 113,000 miles and ran sweet otherwise until that happenned--its in the scrap pile now..

and all that unburned fuel has to go somewhere--usually washes the oil off the cylinders,and goes into the crankcase--both very detrimental to the engine.
