Good truck rifle options include the following:
Ishapore SMLE MK III 2A 7.62 rifle or scout rifle conversion with stripper clips and spare magazines fit with flash hider or muzzle break and ashley scout scope mount and 1-4X Burris EER postol scope or red dot sight hollow fiberglass stock filled inside with survival type gear.
SKS Yugoslavian with Tech Sight rear and Williams front fire sight Chinese chest pouch filled with stripper clips. Great low budget choice!
Sporterized 1903A1 or 1903A3 Springfield .30-06 fiberglass stock Lyman rear and front sights shorten and recrown barrel Galco butt cuff and ching sling.
These rifles turn up as sporters at gun shows and at places such as
www.auctionarms.com www.gunsamerica.com www.gunbroker.com
Mosin Nagant 1938 carbine/rifle cheap to buy and ammo is cheap and easy to come by the action is slower to work and reload but it is a rock solid rifle and is very handy. Great low dollar rifle!
Swiss K-31 makes a great truck rifle it is one of the fastest manual operated rifles ammo can be hard to find and spare magazines are expensive they are tack drivers and won't be cheap much longer. Check into a Mojo peep sight and a no drill scope mount
www.surplusrifles.com
Also one alternative is a good used Remington 742 760 7400 7600 just make sure it functions and cycles reliably and always keep the chamber clean. Barrels can be shortened to 18 inches and a folding stock such as one from Choate Machine and Tool can be modified to fit and reliable 10 round magazines are available from Cabela's they are made by Triple K and are all steel. Check what the overall legal length limit is first and have a unique trunk rifle or keep original Walnut stocks for a low profile rifle.
Enfield MKIV .303 sporterized with a short barrel and trimmed stock these also turn up from time to time already hacked up. They can be made into nice light weight handy little carbines stripper clips and an extra magazine or two and you are good to go.
Any Enfield can be lightning fast to use with a little practice do a you tube or Google video search under mad minute for a bit of background info.
The fastest way to work the rifle is to make like you are going to give someone a birdie use your middle or birdie finger to pull the trigger and place the ball of the bolt firmly between your curled trigger finger and thumb and work the bolt using the thumb and trigger or index finger to hold and cycle the bolt firing with the middle finger as the trigger finger. After practice it is very fast!
If possible try to borrow or fire each of whatever rifle you may be considering at different ranges.
Best of luck.