As yellosub54 says: With your connections in the LS world, you should be able to find an unrepairable LS block. As you know, they are aluminum. Cut out the webs between the bores and maybe the journals. Create some adjustable end plates or adjustable motor mounts on it that just sit on the frame/clamp to frame. After hogging it out, it should be reasonably light. It will be exact and have threaded holes.
The lightened block can also be held "floating" on a wheeled, adjustable lift stand. It would then have all axis (X,Y, Z and pinion angle) adjustment from underneath.
On the other hand, the P-AYR are accurate and have many bolt holes. They have transmission mock ups also or an aluminum transmission core can be used and lifted up. Most of the time, the exhaust is not a feature used to locate a motor. the motor is located by more important factors. Then it's up to the swapper to find or build exhaust after motor is in or located. You will always have some extra work to do but fitting motor to firewall, it's X,Y,Z, pinion angle, fan clearance, motor mount locations and tranny clearance are the important issues, not exhaust. After you have solved these issues and cleared steering parts and suspension parts, you can't start moving the motor to accommodate exhaust. You can hand hold stock exhaust manifolds or headers in place to get an idea of what mods will need to be done for exhaust.
I know, you know all this already, just saying maybe sit back and decide what you can do and what you can leave as doable.
