Seems odd that you're having problems with that. I ran the same setup for years and years in Utah and it worked very well. Is your fluid heat riser the first thing in the lineup, meaning the hot coolant comes out of the head and then goes under the intake, then back into the heater inlet? You might want to check your choke operation too. When I went to Chevy power, I ran Vortec heads which have no exhaust crossover to heat the carb as these were only used on fuel injected engines, but mine was carburated. I made an air cleaner with two inlets, one on each side from soem junkyard remnants. The dampers are controlled by vacuum, so I plumbed a small line off the brake booster hose into the cab, through a valve that I could control, and then back to these dampers. I clamped some conduit of the proper diameter onto my headers and ran some of that goofy silver flex tubing up underneath the air cleaner to the bottom of the dampers. Lastly I installed a bleed valve inside the rig, between the valve and the dampers. When it was cold, I would open the line and close the bleed valve allowing vacuum to close the dampers so the air cleaner was pulling hot air from both sides off the headers. When the engine was warmed up, I would close the valve and open the bleeder so the dampers would open and allow cold, unrestricted airflow into the air cleaner. Worked flawlessly and I could run it all year without affecting performance. Maybe something like that could help you.
