IMO, I wouldn't unless it was my last option. I just hate to do any mods that I can't put back. My core support only covers about an 1 1/2 inches on each side. I feel that the extra surface area of the wider radiator helps, just not as much as if the core support opening was wider. I think a good mechanical fan, that pulls a lot of air, coupled with a good fan shroud, would help the op the most. I also have the 69 340 swinger in my avatar, and installed a 22 inch aluminum radiator, since I couldn't find a correct brass one for a reasonable price. I had all the same symptoms as the OP. I fabricated my own shroud, since the stock one wouldn't fit, and all my troubles went away. No more low speed overheating, never runs hot. And the hood to core support seal really helps.
