My solution is to use PO's Y pipe, add a 90 to the left, add pipe and a 180 to go to the left side of the engine bay then feed muffler which is a universal tractor unit with main body length of less than 12 inches. Secure muffler with rubber mounts to allow motion to accomodate any engine motion. The "TAILPIPE" will be another 90 aimed at the OEM hole thru the door. Whole new assembly will be welded and appropriately clocked toward the OEM hole. Will not extend thru the door. Possibly add a deflector on the door, possibly oversized pipe. This will aim exhaust to rear and make cab area a little quieter while prevent rain entry. Comments please.
Sounds like it should work, I would be interested in seeing a picture.
The concern that I would have would be with attaching the exhaust mounts to the frame. I know you said that you will be rubber mounting the system, so mounts with high degree of compliance would be advised to prevent the exhaust from cracking. The entire engine and pump assembly is rubber mounted in the frame and can move, your mounts will have to do the same, and will have to absorb both high and low frequency vibration.
There also could be a concern of having that much exhaust plumbing in the engine compartment, rejecting heat, raising the engine compartment temperature.
Ideally, you also want the exhaust exit as far away from any point of entry in the cab as possible, to keep exhaust gases away from the operator. If you do not have a window in the back of your cab, you may want to consider installing one. I put one in mine out of an S70.
That is my $0.02. I am really interested in seeing what you come up with.
Cheers - SR