Here's a pic of it mocked up just for fun.
The magic marker lines are where I plan to drill, but I just don't want to go drilling into it until I know what I'm doing...

with just some quick calculation to get approx the same area of opening as a single 1.5" diameter you would need to add six 1/4" holes with the 3/4" single hole to equal the same open area

anytime,  when your dealing with tube, ya gotta figure the area of what you want compared to what you have, then go from there, It doesn't really conform to just adding what would equal the same diameter such as 2 3/4" wouldn't flow the same as 1 1.5" tube.

R.F.

I'd try it like it is first... It's only gonna' get louder as you drill...

Practically speaking, I like Serowbot's idea of trying it as-is first, and then maybe do small increments of drill-listen-drill-listen..  

I know that if I don't drill it out, the bike will run like crap if it runs at all. 3/4 diameter opening for the exhaust on this bike is way too small; that's half the size of the rest of the header pipe.

I drilled it out with 6 1/2" holes to start. That seems to make sense to me. It'll be a couple of weeks yet before she's running; still needs all of the wiring redone and routed, but if I need to go bigger then, then I will.

It's going to be loud no matter what simply because there's no glass packing what-so-ever in that thing. The muffler has a 3/4" perforated tube running down the length of it inside; originally the space around that tube would have been packed with glass to quiet the noise. By drilling holes into the outer ring of the endcap, and around the bottom end as well, where it fits into that exhaust pipe piece, the exhaust will flow through both the outside chamber and the inside chamber. I expect this will act similarly to having a steel baffle in a straight pipe, but who knows!! I'll have to wait until it's running to find out.

(Might have to look for an alternative to pass inspection with!!)
This may end up as bench sculpture, or lawn art, but hell, I had the muffler already, and that piece of exhaust pipe only cost me $5...

Thanks HondaLavis.

I've already drilled six .25" holes, but you're saying I need 3 more.
Visually speaking, I can't add more holes, but I can make the six holes I have now larger. What size should they be?

At this point it's all guesswork, (I actually ovalled the holes out a bit with the drill since I was drilling into a 45 degree surface anyway) so I am going to leave it at that for now. once I get it running I will decide how it's working as we get it tuned up.
I can certainly drill the holes out larger if I need to, but the cool thing about this little mini-muffler is that it's made to be packed, so if I go too far and need to restrict it some more, I could open it up and stuff some stainless scrubby pad material inside to both quiet it some and to slow down the flow, so I'm not too worried.
Regardless of the tuning, it may end up way too loud to pass inspection anyway, in which case I'm back to square one; but hopefully it'll do the job.

I'm going to be welding a small 4" section of baffle into the connector pipe as well, so that should help, (That was Lancer's suggestion to me a while back.) and there's room in there to squeeze another 4" piece as well if I want to do that, so I think I have a few options here...

No matter what, I'm expecting it to sound pretty mean....