It's no secret I'm a SBC fan! I think it's great he's seeing those kind of speeds with that boat.
I'm on my second boat with twin SBC power- first (still own it) is a '86 F242LS Formula with Alphas- 355's I built myself that runs 68.7 GPS fully loaded with full fuel (100 gals) and weekend overnight gear, 2 people.
The new toy is a '99 Active Thunder Savage- stock 350MagMPI's/Bravos that runs slightly over 70 fresh water, 73 salt.
I don't doubt those speeds on the Scarab- the newer technology in building these boats has lightened them up a lot! I haven't personally weighed the two, but going by mfg. specs, my 4 foot longer AT is over 1000# lighter than my Formula! (there is NO wood in the AT anywhere)
There are stroker/blower SBC's in the AT's future.
Congrats to your friend!