Try some easy things 1st, pull the thermostat see what it does worse or better, my caddy motor has a small rad in the t bucket and wanted to heat with the thermostat in it ,took it out and it helped a lot, also is a easy thing to try , maybe a problem thermostat now.
2nd pull the hood and see what it does at speed, worked fine before so what happened to change the setup, the hood? any way of mounting hood so some air can eascape?wrapping the headers may help but thats only underhood heat which may not affect the overall heating problem