59A is 100hp. Quick and dirty, you aint going to get a "lot" of hp increase without a major build and considerable expense. You need to give us more info.... what sort of budget you envisage being the major info, also what is it going in, and what is the drivetrain?
Twin carbs, twin exhaust and milling the heads... you might get 110-120hp. Add a cam and adjustable lifters... another 10hp.
From there you're into boring out and oversize pistons plus a longer stroke crank to give you more cubic inches. Speedway and others sell a kit with stroker crank, large pistons and new rods for around the two grand mark. Machining on top of that. Mods are cumulative... eg: twin carbs on a stocker wont give as much hp increase as they would on a worked engine.
If $$$ is no object a blower is the way to go, there are some guys on here running blowers. That, plus the increased cubic inches will get you anywhere from say 160hp to 250hp, depending on a lot of factors.... one being, how tractable does it have to be? So many variables....
If it's just an old truck you want to go a bit faster, stick with minor mods and upgrade the trans for better shifting and diff ratio for better highway speeds.