light compared to what?an iorn 727?they are heavy and drink power.
if given the choice of a mild 383 with a 518 and a hotish 318 with a 904 i think the gas mileage would be close and (remember 323 gear) and the small block would beat it around a road course.

the right 518 are getting hard to find (no electronics)

id bet your friend has alot of cash in that 700r4.ive had friends shred thos with 305s.

---

The 518 and the 500 with the OD are both heavy in
relationship to the stock 727 or the 904... the OD
is heavy... the case is a fair bit longer on either
of the OD units... on his 700R4 we did it our selves
and it doesnt has a lot of fancy stuff in it... the
biggest thing was getting the valve body right(which
took us about 4 or 5 times to get it right)... but
he says now if he changes over to anything else it
would be the 200R4
NOTE
I believe the OD unit is about 40# heavier than the
stock version 727.... but I needed the OD to run on
Drag Week to keep the revs down and to get better mileage