since it's your first foray and you're budget limited, i would strongly suggest that you stay small block.
while the (possibly) 4K price tag on a big block looks super enticing, all of the peripherals needed to install it will absolutely bury you. to get that thing shoehorned in there you're gonna need motor mounts ($200), a radiator or have your modified ($$), a transmission ($$~$$$), shorten your driveshaft ($), exhaust ($$$), some wiring work (inconsequential, DIY) and that's just to make it fit in there. [$=hundreds, $$$= 1K+]
then, you'll need to figure out if your steering all works (it won't) and your brake master cylinder(you'll probably have to swap something), maybe remove your heater blower motor and wiper motor.
then you'll need to be addressing the brakes and suspension to make it safe. if the suspension needs a full rebuild you get the extra treat of having a one year only idler arm that's expensive to go along with all the 67~72 parts that are also spendy.
and that's not even counting upgrading disc brakes, a rear end, springs, shocks...
that 4K for a big block could get you a decent little small block and back on the road pronto and you'd have some spare monies left over for other upgrades.
