Just a few notes from my perspective. I considered this along the way as well.
2.5 * Whatever form....bit of a dud (no offence)
3.4 * Iron head..nice upgrade (Tyler has one) I have most of the 2.8 add on bits you may need.
3.4 * TDC or DOHC nice but I think it's just asking for $$$ probs.
3.1 * S/B an easier job and they're everywhere. 3 in Autotrader cheap.
3.8 * Quality upgrade in any form but more costly in time and $ to install.
4.3 * Need an expensive kit or lots of fabbing.
Quad $ * Nice but may be hard to locate in good shape.
4.? Caddy * Nice and fairly easy with auto tranny. They say there are cheap motors out there but I can't fnd a good one. I may know where there are a couple of 4.5's. I believe you need a custom flywheel US$400+ for a manual tranny.
N* * Call your banker.....'nuff said.
I have a new 2.8 long block that I may sell or trade for a good Caddy if I have a blonde moment. I have about 2300.00 in it and it's still on the stand.
I would go 3.1 I think. There are instructions around for using the newer DIS version. Won't look like a 2.8 but if that's not a problem, so what.
Gary