THM 400 to 700 R4 conversion

[Rnd]

I just finished up the paint and cosmetic work on my 1977 GMC 20 Crew Cab (see pics thread).
Took it out for a 60-70 mile ride and no surprise, the fuel mileage sucked and it turned some relatively high rpm's on the thru-way.

I'm thinking of swapping out the 400 trans. and installing a 700 R4.
I see Monster Trans company has a conversion "kit". Has anybody done this?? If so what should I be looking for in the way of unforeseen expenses??

Tech. info:
454 stock (for the time being)
400 trans
4:10 floating rear axle
2 WD

Any thoughts or input from anybody that has first hand experience would be greatly appreciated.

Thank you,