Hey back, I have the same deal as you. I own a 71 & 72 240z's and want to v-8 one. I got a quote of $12,000 turnkey to put in an LS1 350 shortblock. That's in Dallas, Tx. As far as buying the 240....paid $1850 for the 71 in 1995 & just paid $2400 for the 72, but the 72 spent 35 years in a barn so little rust. The biggest thing on a v-8 conversion is if the engine metal has rust, don't but it. Not for a v-8 conversion.
I just talked to a man who doe's this and he said if there's rust, you need to pass it up.
I need lots of help here & money
Good luck, Donnie