Misfire and backfire on first start up

Just to be sure this only does it on cold starting? So, when it warms up it does not have this issue?

If that is the case, then we need to jump straight to a vacuum leak.

Here is a guide that will help us get started on this:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

The 454 was well known for vacuum leaks so we need to focus on this.

Next, we need to check for codes even if the check engine light is on.

https://www.2carpros.com/articles/buick-cadillac-chevy-gmc-oldsmobile-pontiac-gm-1983-1995-obd1-code-definitions-and-retrieval-method

Let's start with this and we can go from there. There is a chance this is an actual misfire, but we need to start here.

Thanks