Yeah, there's a reason why they make the adjustable sections. It's because the pipe isn't supposed to be cut.

Thanks for the response Xanndra.

With a cooler head, today, I got back up there and re-assessed the situation. I took the finished ends that I cut off and they fit onto the cap very easily. The 8" pipe seems to slide all the way on as far as I need until it hits the outside plate. The 5" only wants to go on until finished end "ends".. or about 2.25". With all that said, the distance from the 90* elbow to the start of the cap pipe is 18.75". It appears the interlock overlap is about another 2.25... or 21" total pipe length that I need. Although Simpson doesn't make a 21" pipe, I think I can use (2) 6" + (1) 12" to get that 21" pipe (6" + 6" + 12" - 2 * 1.5" overlaps). So, tomorrow, I'm headed to the local chimney store to see if those three pipes will work. I'm hoping so.

I could also use the adjustable length pipe to get 21", but Mendota states repeatedly in the manual not to use adjustable/variable piping. I think that's the most frustating part... Simpson and Mendota have opposite views/directions on a number of things... from where you should/shouldn't use pipe sealant... to whether adjustable pipe is OK.

Thanks again.

You should always follow the appliance manufacturer's installation instructions first. The vent manufacturer's instructions are very "one size fits all," whereas the appliance manufacturer's are specific to that appliance.