The process isn’t hard. You are not trying to duplicate a specific angle right. I recently did some exhaust ends for a buddy of mine. I cast blanks out of aluminum then turned to my likings. Here’s two that I did first is similar to what you want. 
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Heavy taper, help with velocity stack how to.
- Thread starter GunsOfNavarone
- Start date
- Joined
- Jun 26, 2018
- Messages
- 1,733
Nope. No specific angle needed. Your 2nd picture is about there, just shy of the roller lip, which is particularly challenging. I think I'm going to try the map gas with the template technique. Seems pretty easy....sure it won't be!