I've been looking at some EGT sensors and input boards for the Megasquirt as well. I am considering drilling the exhaust manifold for the EGT sensors and retaining the exhaust manifold for the turbo.
I am hoping for enough resolution on the EGT's to be able to do some individual cylinder fuel trims to balance out the fuel in each cylinder and just have a single O2 sensor to know I'm still in the general AFR ballpark. When turbo'd it would allow me to set up some safety systems if a cylinder gets hot (lean out) or goes cold.
What's the furthest from the head some of you have mounted a turbo? I'm thinking I will route the single 2.5" exhaust over to the passenger side of the car via the space just past the torque converter and back up into the passenger side of the engine bay. It'll be quite a run of tubing. I'll see if I can come up with a better solution once this engine is in the car and can see what room I have to work with.
Rebuilding an OHC Pontiac 250 with EFI and a Turbo