A Ford 200- 250 I6 are just a little longer than a SBF, if you wanted to maintain a wheelbase of about 100" which most with V8's are, this would be the way to go. If you wanted bigger, you could get a 300 Ford, there a lot longer but they make a lot more torque. A early Jeep 4.0L would work good with a carb manifold, you can stroke a 4.0 with a AMC 258 crankshaft, which makes it about 4.5L. If you wanted, you could use a Mopar Slant 6. they can make great HP numbers, both N/A and Forced induction, a stock 225 Slant 6 with a correctly sized turbocharger makes about 1hp per cubic inch running minnimal boost, and some built up slant 6's I'v seen turbocharged make upwards of 600 Hp. I'v seen slant 6's in a few hotrods.