Just a question about magnum swap

I did a carbed 5.9 Mag & 518 in my 77 CrewCab with 4.10's that got a solid 9-10 mpg with it's stock 360/727 & had a top speed of about 75
When I was done it had twice the power,18-19 mpg hwy & top speed over 100 easy
I switched the converter & OD using toggle switches

It got 3 mpg hwy better than the 97 SWB 2WD that the engine came from

I leaned the carb out & got 21 mpg hwy but would burn the exhaust gaskets out on really long trips