Hi Ron!
I'll try to answer all your questions the best I can.
1. Aristo makes 3 switches, non of which are 8' diameter. There is a "curved" switch that match 4' diameter, the "Wide Radius" switches that match a 10' diameter, and the #6, which is a prototypical switch with a 1 in 6 diverging angle.
2. I'll assume you have the Wide Radius switches.
3. It sounds like you have back to back WR switches in a crossover. Not recommended, makes a nasty S curve. (Yes I know there are people out there that make it work. It's still a nasty S curve).
4. Stock frogs from Aristo are not deep enough on the WR switches, especially if you trim the frog top down to the rail top height.
5. The new replacement frogs are better ($1 each from Aristo), and have the NMRA recommended 3mm depth.
6. Normally the flanges on Aristo steam locos are less than 3mm in "depth", but check yours.
7. I find your statements about having to cut the brass rails to merge to the frog VERY worrisome. The new frogs fit exactly. Hmm... maybe you DO have the "tight" turnouts, the 4' diameter. No one has ever said that the new frogs do not fit exactly. You do have to remove the screws underside to pull the "Stub rails" back to clear the frog.
8. All Aristo locomotives are tight gauge, measured with an Aristo gauge (which is correct). Yes, virtually NO WAY to correct the gauge. Weak point in the Aristo design. The only way to fix this "correctly" is to completely disassemble the motor drive system and insert a shim between the axle and the gear housing the axles are screwed to. Royal pain in the butt, I know.
9. My ideas for you:
Sell the loco instead of trying to fit the new motor block into your older Pacific. It can be done, but it's not pretty in my opinion. There are some pictures of this here or on the Aristo forum.
Double check that you do not have the 4' turnouts. If so, throw them away. Way too tight.
Yep, those are my opinions, although you stated you did not want to get a whole new engine.
Regards, Greg