Hmm I dunno, each situation is different. I live in the deep south, where it doesnt get very cold. The heat riser didn't do much for me, but you may need it if you drive in colder weather. My riser was stuck, so when I had the exhaust manifold off, I cut it out with a grinder. Then I drilled and tapped the riser holes, and plugged them with 2 short bolts.
The intake and exhaust manifolds do bolt together, so Id imagine some heat transfer happens anyway. Now for the aftermarket fenton setup, exhaust or water heat is a must.
In your case, Id prolly put the spring on and run it like it is. Unless the exhaust leak can be heard over all the other noise while you're going down the road.
These old machines made so much darn racket when new. My import-driving buddies think my ride is going to blow apart at any second. (And thats before my 235 started knocking)
Chip
