Gas Mileage???

Ok then
Fwiw
I reworked the numbers; I assumed the following: a bore of 4.03,stroke of 3.58, a Zero deck, a .039 FellPro gasket, 5cc eyebrows,and the aforementioned 60 cc heads. This gives us a 748.31 swept and a 73.9 chamber, and thus a 11.126 Scr.This might work with an ICA of 64* to net a Dcr of 8.89/182psi .Now,a very similar chamber size can be had with the pistons down .010, and a gasket of .029. or down .019/ and .020 gasket; just so the two add up to .039. Also Quench can be increased to about .050, before losing it's effectiveness, so there is the option of adding another 010 to either the deck or the gasket.The quench distance to stay out of has been preached as .060 to .80. I cannot verify that.
--I then built a theoretical cam around that 64*ICA and it looks like this; 264/272/114, in at 112. The overlap would be 40 degrees which is 2* less than the 256 you earlier chose. If you can find, and ordered something like this, in a solid flat-tappet or a roller, you could probably get some pretty fast ramps, such that the 050 numbers might approach;225/233/114 and that is fully 2 cam sizes larger than the previous 212/218/110. The 225 cam is a Ton-O-Fun on gas, making tremendous low-rpm torque.And the 114LDA is very automatic transmission friendly.
With a FT hydraulic, you might be stuck with 220/228/114, which is still 1.5 cam sizes bigger.And will still be a Ton-O-Fun.
The penalty for the bigger cam, might be on the power stroke, which is 4* shorter. This I suppose might cost you a tiny bit of fuel mileage at steady state speeds. But I have cruised a nearly identical combo to yours, down to 1588 rpm at 65 mph, where it returned me 32mpgUs, on 87E10.
I have no idea if this is helpful with propane.