Telly, a friend of ours put one in a RX-7 along with a tranny that had a long 6th gear (forget which) and kept R's low and was getting close to 30mpg.
My Brother has a 240 with an LS1 in it with lots of work on it and still gets good mileage.
I put a 5.3 in my 60.. It's standard block truck motor out of an avalanche and it does wonderfully. Vapor lock or cavitation before the pump is not an issue as long as you build the fuel system right. lots of misinformation out there as far as what an acceptable fuel system is i think. An inline pump with a filter before it just won't cut it when the fuel is hot... hi performance pumps that move a ton of fuel make it worse.