Thanks for that great input. The DC is somewhat low end I suppose, but should work for my purposes until it gives out. I bought it in 1997. I also plan to fit it with a Wynne filter and an Oneida super Dust Deputy separator.
I checked with Grizzly and they do have a replacement inlet that will take me from 5" to 6" and they said there were no technical issues related to the upgrade. I understand the issue of costly fittings and I also like the Oneida fittings. It won't be cheap, but after years of being too busy to do any serious woodworking, things have changed (for the better), and I am looking forward to getting back into the hobby. And with that, I need to get better dust collection. I already knew that, but when my 17 year old son came in the other day and said, "Dad, you gotta do something in here. I can taste the wood when I walk in," he reempasized my believe that it was time to suck it up and spend some bucks.
Here is a post I made on Phil Thien's forum. It was in response to some discussion and questions from a member who was considering upgrading from 5" to 6" mains. I realize in your case, you are starting from scratch, so the cost analysis is different for sure, but from a practical standpoint 1-1/2 and 2 HP DC's reach a point of rapidly diminishing returns at 5". The response below will help quantify that.
"I believe 5" plumbing is all you need for most dust collectors up to 2HP. There may be some exceptions to that rule, but not many. So, to help you folks who are considering spending good money and time to replace 5" pipe with 6", I did a quick little exercise to put the hoped for improvement into perspective.
I created a hypothetical system with 3 ft. of flex hose at the tool end and another 3 ft. at the DC end. In between, I assumed a run that includes 20 ft. of straight pipe, four 90 degree ells, and one 45 degree ell. The SP loss for that system with 5" pipe is 4.35". If you replaced everything with 6" pipe, the SP loss drops to 4.14". That's a 2% gain in SP. You will never feel or see that difference in performance without a test instrument.
Using a typical fan curve, that SP gain will translate into an additional 50 CFM or less. You might be able to see a performance change of several hundred CFM, but not 50!
You probably have a different piping configuration than my hypothetical system, but I suspect running the numbers will yield nearly the same results.
And there are other issues to consider. If you only have a 5" outlet on your blower, then I'm not sure there is anything to be gained by going to 6" on the inlet side. Also, when you go up in pipe size, FPM's drop, and if you can't maintain a velocity of at least 3500 FPM you risk having dirt settle out in your mains.
Another word of caution. A lot of DC manufacturers give you a "Y" that splits the nominal inlet into two smaller sizes. That does not mean the system can handle two lines of the smaller size simultaneously. So, two 4" lines running from a 5" DC inlet are not going to be very productive. I doubt anyone is actually doing that, but the same comment applies if you are splitting your 5" line into two 4" lines at the machine end. "