Making my 396 more streetable???

I have a 1969 Camaro with a 396,bored .30 over to 402 cubes,forged pistons and crank,stock ported oval heads 2.19-1.88 valves,595 lift on intake and exhaust lift Competition cam,Edelbrock Torker 2 manifold,750 Holley double pumper,Msd 6AL ignition,4:11 rear end gears,28 inch tall tires in the back,150 horse power nitrous system,4 speed Muncie M-20 trans.The car runs great on 93 octane gas.I am not sure what the compression ratio is on the car but if i had to guess it would be 10.0.I know that the stock heads were the 360 horse power heads that chevy made and thats about all I know.The forged pistons may be 10.5.I want to keep a manual transmission and this car is driven only the street.I was looking into a transmission that has a overdrive like the T-56 or Tremec 5 Speeds so when cruising at 60 mph I would be driving at alot less RPM's.I have no idea what my gas mileage is because i have not had the car inspected for years but with my combination I would be lucky to get 10 Miles to a gallon??? Any one know what would be a good overdrive transmission that would fit without to many problems or mods???Also I would like to upgrade to Aluminum cylinder heads.I read about the newer Holley Oval Port Heads.They reduce compression almost 1 whole point and still make 51 horse power even with the drop in compression.I would like to get my compression down to keep out possible detonation although i have never had detonation before because i always put in 94 octane. I would like to lower compression a little so i can get 91 or even run 87 octane and still have the extra 50 Horse power.Anyone know who makes a similar or better cylinder head then the Holley Oval Ports that im talking about??? I believe the combustion chamber was 110cc for the Holley heads.

Might want to consider a Gear Vendors overdrive on the back of your tranny, they work great.

The engine was built a long time ago and i did not build it.The guy who built it for me said that he used a thicker head gasket to reduce compression so it would be better on the street.I have no idea how far the piston comes up in the bore.I have never had a problem with overheating,detonation on pump gas but I did have a problem with engine shut off.I don't know if it was the MSD ignition but the car would not shut off if i took the key out.I later fixed it by starting the car and while the car was running flip a toggle switch to cause the alternator to charge the battery.That toggle switch is a switch that turns the charging system on and then can be turned off after the engine is running. I never measured cranking compression.Would that give me a estimate on what my final compression is??? I have never heard of Gear Vendors Over Drive on back of a Muncie-M20.My Muncie M-20 sometimes jumps out of gear so I would not use this transmission to upgrade.How does that work and how much money for there overdrive???I know that Richmond makes a 5 speed and 6 speed but they are really expensive.I know some guys are using the 5 speed Tremec and T-56 6 speed trans in there low geared big block car today???