Best fuel efficiency

[Jarvis-69 GMC 1500CK]

Hello!

I am trying to decide where I want to go with my truck (69 GMC 1500 longbed) (I want 25-30 MPG with 2-300 HP, nothing crazy I am for sure going to be putting in a 5-6 speed manual which ever option i go with)

Here are my options that I am toying with - One of them might get me shot...but oh well!

Option #1 (I think would be the easiest).
Keeping my 80's 305 in it, Efi'ing it, head swap for a vortec etc... tuning it and really squeezing it for all it has (I do not wish to swap to a 350, if I do a swap it is to one of the other 2 options). If I do this what is the max I could see, again 5-6 speed manual and taller rear end gearing

Option#2 modest power diesel swap - just throw away the 305 and just put a diesel in it, if so again what numbers would I see with 5-6 speed

Option #3...this is the one that may get me shot and is kinda fantacy
2Jz swap (straight 6 boosted toyota power plant from a supra)

I have looked online and I see a lot of talk but I cannot find alot of numbers, especially for #1, that is the one I am most likely to do if I can get that 25-30 with EFI and head swap - etc

Also about 80% of my driving is highway so that 25-30 MPG number I desire is highway, I am not silly enough to expect that as city for even combined!