Nice old truck Russ.
I have a Chevy 250 I6 waiting to go into a 70 El Camino for when my son is on Ps. You can wake them up a bit. Mine has an Offenhauser manifold with a Holley 465 4 barrel. You can get headers, cams etc. for them from Clifford Performance. I would leave the I6 in there - suits the character of the truck.
1970 Ford Mustang Sportsroof 302W/Toploader
1970 EL Camino 350/TH350 - The FrankenUte
2006 Jeep SRT8 - Big dumb US fun.
2007 FPV Force6 - Wife's car