If your donor was built in 1967 or after you must have at least 3 point seat belts fitted or you'll fail the MOT and get a £60 fine if you get stopped while driving it. Pre-67 you can use lap belts, diagonal belts or no belts at all.
http://www.rospa.com/roadsafety/info...lt_history.pdf
No belts is a bit like unprotected rumpy pumpy with an exotic stranger you met in a hotel bar abroad: a tad more exciting, perhaps, but likely to have unpleasant consequences.
Diagonal belts are a little better but seat occupants tend to 'submarine' during frontal impact, sliding under the belt and ending up squashed in the footwell.
Lap belts cost US car manufacturers $billions in compensation claims in the 60's and 70's due to 'seat belt syndrome' as they caused a peculiar folding of the upper body on impact that separated the lower spinal vetabrae, cut the spinal cord and left the seat occupant paralysed from the waist down.
It's your choice but I'm fitting brand new three point inertia belts.
And a condom, just in case...