What a 258 can handle (soon to turbo)

Mid 70s 258s were 8:1 compression -- 72-78. 71 models had 8.5:1.

I found one article with a 71 Gremlin with a 258 running 8 psi. I wouldn't run that much, only 5-6 with stock pistons and trans, but he's been running fine for years (and states that the trans is a weak point, but uses a BW M4x that was made for a 304). States he has no spark control, but runs premium fuel and a Holley 500 2V carb. This is a "semi remote" mount (turbo on right side with pipe run from exhaust manifold under engine) blow-through setup and what I'd recommend -- has to be the easiest way to do it.
http://gremlinx.com/1971-turbo-gremlin/

Found a "Hot Rod" magazine article on the 1977 Randal AMX turbo six. They test drove the prototype 10 psi boost car. They state the stock trans wasn't up to 10 psi boost, so Randall dialed the "production" setup down to 5 psi -- 3 psi was in by 1800 rpm, all five by 2500. Randall modified the end of the stock exhaust manifold to mount the turbo flange on the left side, and a draw-through Motorcraft 2V (370 cfm -- 1.23 venturi is 356 cfm, closest, next larger size is 1.33, rated at 424 cfm). The article does say the carb was modified, but a 356 cfm should do the trick. Seems the 77 kit is the only Randal turbo six offering, 1977 only.
http://clubs.hemmings.com/classicamx/turbo-hornet/turbo-hornet.htm